Class 12 Maths Applications of Derivatives – Step-by-Step NCERT Solutions

NCERT Maths 12th 2023 ( Maths Ncert Solutions class 12th )

Pallavi Pathak
Updated on Aug 4, 2025 17:05 IST

By Pallavi Pathak, Assistant Manager Content

Application of Derivatives Class 12 covers how the derivatives can be used to find the equations of tangent and normal to a curve at a point, to determine the rate of change of quantities, and to find turning points on the graph of a function. The chapter covers the applications of the derivative in various disciplines such as Science, Engineering, Social Sciences, and other fields.
Application of Derivatives Class 12 NCERT Solutions covers the increasing and decreasing functions, the rate of change of quantities, and maxima and minima. The subject matter experts at Shiksha have created the NCERT solution to help students deepen their understanding of concepts. It is ideal for exam preparation. The detailed and step-by-step solutions help students to score high in the CBSE Board exam and competitive exams like JEE Mains.
To get the NCERT Solutions of Class 11 and 12 of all three subjects, including Chemistry, Maths, and Physics, check - NCERT Solutions Class 11 and 12.

Table of content
  • Quick Overview of Applications of Derivatives – Class 12 Maths
  • Application of Derivatives NCERT Solution PDF: Free Download
  • Class 12 Maths Application of Derivatives: Key Topics, Weightage
  • Important Formulas of Application of Derivatives Class 12
  • Class 12 Application of Derivatives Exercise 6.1 Solutions
  • NCERT Solutions for Class 12 Maths Ch 6 Application of Derivatives- FAQs
  • Class 12 Application of Derivatives 6.5 Solutions- (Old NCERT)
  • NCERT Class 12 Application of Derivatives Miscellaneous Exercise Solutions
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Quick Overview of Applications of Derivatives – Class 12 Maths

Here is a summary of the Applications of Derivatives Class 12 NCERT Solutions:

  • The chapter covers the rate of change of quantities and the chain rule.
  • It also includes the First Derivative Test, comprising the concepts of local maxima, local minima, and point of inflexion.
  • It covers the Second Derivative Test and the working rule of finding absolute maxima and/or absolute minima.

To get access to the NCERT Solutions of all chapters of the Class 12 Maths with free PDF, and important topics, check - Class 12 Maths NCERT Solutions.

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Application of Derivatives NCERT Solution PDF: Free Download

Students should download the Application of Derivatives Class 12 PDF from the link given here. It will help them in getting concept clarity and scoring high in the CBSE Board exam and in the JEE Main exam.

Class 12 Math Chapter 6 Application of Derivatives Solution PDF: Free PDF Download

Try these practice questions

Q1:

For the function f(x) = 4 loge(x – 1) – 2x2 + 4x + 5, x > 1, which one of the following is NOT correct?

View Full Question

Q2:

If the tangent at the point (x1, y1) on the curve y = x3 + 3x2 + 5 passes through the origin, then (x1, y1) does NOT lie on the curve:

Q3:

The sum of absolute maximum and absolute minimum values of the function f(x) = |2x2 + 3x + 2| + sin x cos x in the interval [0, 1] is :

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Class 12 Maths Application of Derivatives: Key Topics, Weightage

Application of Derivatives Class 12 is a part of calculus in JEE Main exams, which is a significant area of the Mathematics section. See below the topics covered in this chapter:

Exercise Topics Covered
6.1 Introduction
6.2 Rate of Change of Quantities
6.3 Increasing and Decreasing Functions
6.4 Maxima and Minima

Application of Derivatives Class 12 Weightage in JEE Mains

Exam Number of Questions
JEE Main 2-4 questions

Related Links

NCERT Notes for Class 11 & 12 NCERT Class 12 Notes Class 12 Maths Notes PDF for CBSE Exams
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Important Formulas of Application of Derivatives Class 12

Application of Derivatives: Important Formulae for CBSE and Competitive Exams

  • Rate of Change: d y d x \frac{dy}{dx} , where y y is the dependent variable and x x is the independent variable.
  • Slope of Tangent: m = f ( x ) m = f'(x)
  • Slope of Normal: m = 1 f ( x ) m = -\frac{1}{f'(x)}
  • Equation of the tangent: y y 1 = f ( x 1 ) ( x x 1 ) y - y_1 = f'(x_1)(x - x_1)
  • Equation of the normal: y y 1 = 1 f ( x 1 ) ( x x 1 ) y - y_1 = -\frac{1}{f'(x_1)}(x - x_1)
  • Approximation Formula: f ( x + h ) f ( x ) + h f ( x ) f(x + h) \approx f(x) + h \cdot f'(x)
  • Condition for Increasing Function: f ( x ) > 0 f'(x) > 0
  • Condition for Decreasing Function: f ( x ) < 0 f'(x) < 0
  • Local Maxima/Minima (First Derivative Test): Solve f ( x ) = 0 f'(x) = 0 and analyze sign changes of f ( x ) f'(x) around the critical points.
  • Conditions for Minima and Maxima:
      • f ( x ) = 0 f'(x) = 0 and f ( x ) > 0 f''(x) > 0 for local minima.
      • f ( x ) = 0 f'(x) = 0 and f ( x ) < 0 f''(x) < 0 for local maxima.
  • Second Derivative Test: Use f ( x ) f''(x) to confirm the nature of critical points.
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Class 12 Application of Derivatives Exercise 6.1 Solutions

Class 12 Application of Derivatives Exercise 6.1 discusses the fundamental usage of Derivative techniques to solve various kinds of problems. Application of Derivatives Exercise 6.1 focuses on the rate of change of quantities, increasing and decreasing functions, tangents and normals, approximations, and finding maxima and minima. The concepts discussed in Class 12 Math Exercise 6.1 Solutions will help students get a good understanding of calculus and even provide tools to solve problems in Class 12 physics and other fields too. Class 12 Ch 6 Exercise 6.1 Solutions includes 18 Questions (10 Long, 6 Short, 2 MCQs). Students can check the complete solutions for Exercise 6.1 below;

Application of Derivatives Exercise 6.1 Solutions

Q1. Find the rate of change of the area of a circle with respect to its radius r when

d d r = d d r r2) = 2πr.

A.1. (a) r = 3cm

When r = 3 cm,

d d r = 2 × π × 3 cm = 6π.

Thus, the area of the circle is changing at the rate of 6π cm c m 2 5 .

(b) r = 4cm

whenr = 4cm,

d d r  = 2 × π × 4cm = 8π.

Thus, the area of the circle is changing at the rate of 8 c m 2 5 .

Q2. The volume of a cube is increasing at the rate of 8 cm3 /s. How fast is the surface area increasing when the length of an edge is 12 cm?

A.2. Let x be the length of edge,v be the value and s be the surface area of the cube then,

y = x3.

and S = 6x2, where x is a fxn of time.

Now,  d Y d F = 8 c m 3 5

d d t  (x3) = 8

d x 3 d x · d x d t = 8  (by chain rule)

 3x2  d x d x = 8 .

d x d t = 8 3 x 2 .

Now,  d S d t = d d t  (bx2) =  d ( 6 x 2 ) d x d x d x  = 12x  8 3 x 2 = 3 2 x c m 2 5 .

When x = 12 cm,

d S d t = 3 2 1 2 = 8 3 c m 2 5 .

Q3. The radius of a circle is increasing uniformly at the rate of 3 cm/s. Find the rate at which the area of the circle is increasing when the radius is 10 cm.

A.3. Let ‘r’ cm be the radius of the circle which is afxn of time.

Then,  d y d t  = 8 3cm/s as it is increasing.

Now, the area A of the circle is A = πr2.

So, the rate at which the area of the circle change  d d t = d d t πr2.

= d d r π r 2 · d r d t

= 2πr 3

= 6πr. cm 2 5

When r = 10cm,

d d t = 6.π × 10 = 60π cm 2 5

Q4. An edge of a variable cube is increasing at the rate of 3 cm/s. How fast is the volume of the cube increasing when the edge is 10 cm long?

A.4. Let ‘x’ cm be the length of edge of the cube which is a fxn of time t then,

d x d t  = 3cm/s as it is increasing.

Now, volume v of the cube is v = x3

Ø Rate of change of volume of the cube d v d t = d q 3 d t .

= d x 3 d x d x d t

= 3x2.3

= 9x2 cm 2 5

When x = 10cm.

d v d t = 9 x (10)2= 900 cm 2 5

Q&A Icon
Commonly asked questions
Q:  

27. Prove that y=4sinθ(2+cosθ)θ is an increasing function of θ (0,π2) .

A: 

We have, y = 4sinθ(2+cosθ)θ

Differentiating w rt.Ø we get,

dydθ=ddθ[4sinθ2+cosθθ]

=(2+corθ)ddθ(4sinθ)4sinθddθ(2+corθ)(2+cosθ)2dθdθ.

=(2+cosθ)(4cosθ)4sinθ(sinθ)(2+cosθ)21

=8cosθ+4cos2θ+4sin2θ(2+cosθ)2.1

=8cosθ+4(cos2θ+sin2θ)(2+cosθ)2(2+cosθ)2

=8corθ+4[4+cos2θ+4cosθ](2+corθ)2.

=8cosθ+44cos2θ4cosθ(2+cosθ)2

=4cosθcos2θ(2+cosθ)2=cosθ(4cosθ)(2+cosθ)2

When θ[0,π2] we know that, 0cosθ1.

So, 4cosθ>0

And also, (2 + cosθ)2> 0.

dydθ0θ[0,π2]

Hence, y is an increasing fxn of θ in [0,π2]

Q:  

74.Find the maximum and minimum values, if any, of the following functions

given by.

(i) f(x)=(2x1)2+3

A: 

(i) We have, f(x) = (2x - 1)2 + 3.

For all x?,(2x1)20

(2x - 1)2 + 3 ≥ 3.

f(x) ≥ 3.

∴The minimum value of f(x) = 3. When 2x - 1 = 0--> x = 12

Again as x,f(x) as there is vouppa bound to ‘x’ value hence, f(x) has no maximum values.

(ii) f(x)=9x2+12x+2

A(ii)

We have, f(x) = 92 + 12x + 2.

f(x)=9[x2+12x9+29] (Taking 9 common from each team).

f(x)=9[x2+4x3+29]

f(x)=9[x2+2×2x3+(23)2(23)2+29]

f(x)=9[(x+23)249+29]

f(x)=9[(x+23)229]=9(x+23)22

For all x?,(x+23)20

(x+23)2202

f(x)≥ - 2.

∴The minimum value of f(x) = -2 when x+23=0

And as x,f(x) so f(x) has x=23.

no maximum values.

(iii) f(x) = (x - 1)2 + 10

A(iii)

we have, f(x) = - (x - 1)2 + 10

For all x?,(x1)20.

(x - 1)2 ≤ 0

-(x- 1)2 + 10 ≤ 10.

f(x) ≤ 10.

∴maximum value of f(x) = 10 when x - 1 = 0  x = 1.

And minimum value of f(x) does not exist.

(iv) g(x) = x3 + 1

A(iv)

we have, g(x) = x3 + 1

For the given fxn, x?,

g(x)asx

and g(x)asx

Q Maximum and minimum value does not exist.

Q:  

14. Sand is pouring from a pipe at the rate of 12 cm3 /s. The falling sand forms a cone on the ground in such a way that the height of the cone is always one-sixth of the radius of the base. How fast is the height of the sand cone increasing when the height is 4 cm?

Read more
A: 

Let r cm and h cm be the radius and the height of the cone. Then,

h =16 r. H = 6h

So, volume, V of the cone =13 πr2h

=13π(6h)2×h. 4ddt.

= 12 × h3

Rate of change of volume of the cone wrt the height is

dVdh=ddh(12πh3) = 12 × π × 3 × h2.

As the sand is pouring from the pipe at rate of 12cm35

we have

ddt=12

dvdh×dhdt=12

36πh2dhdt=12.

dhdt=1236πh2=13πh2.

dhdt|h=4 =13π×(4)2=148π.

Hence, the height is increasing at the rate of148x cm/s.

Q:  

63. The slope of the normal to the curve y = 2x2 + 3 sin x at x = 0 is

(A) 3 (B) 13 (C) -3 (D)13

Read more
A: 

Given,  y=2x2+3sinx

Slope of tangent,  dydx=4x+3cosx

dydx|x=0=4×0+3×cos0=3

So, slope of normal =13

Option (D) is correct.

Q:  

99. Show that semi-vertical angle of right circular cone of given surface area and maximum volume is sin113 .

Read more
A: 

Kindly go through the solution

Q:  

122. The line y = mx + 1 is a tangent to the curve y2 = 4x if the value of m is

(A) 1 (B) 2 (C) 3 (D) 12

Read more
A: 

The equation of the tangent to the given curve is y=mx+1.

Now, substituting y=mx+1. in y2=4x,  we get:

(mx+1)2=4xm2x2+1+2mx4x=0m2x2+x (2m4)+1=0.......... (i)

Since a tangent touches the curve at one point, the roots of equation (i) must be equal.

Therefore, we have:

Discriminant = 0

(2m4)24 (m)2 (1)=04m2+1616m4m2=01616m=0m=1

Hence, the required value of m is 1.

Therefore, option (A) is correct.

Q:  

21. Show that the function given by f (x) = sin x is (a)

increasing in (0,π2) (b) decreasing in (π2,π)

(c) neither increasing nor decreasing in (0,π)

Read more
A: 

We have, f (x) = sin x.

So, f (x) = cosx.

(a) when, x ∈ (0, π2) i e, x in 1st quadrat.

f (x) = cos.x> 0

f (x) is strictly increasing in  (0, π2) .

(b) when, x ∈ (π2, π) in IInd quadrat

f (x) = cosx< 0.

∴f (x) is strictly decreasing (π

(c) When, x ∈ (0, π).

f (x) = cosx is increasing in  (0, π2) and decreasing

in  (π2, π) and f  (π2) = cos π2=0.

∴f (x) is neither increasing not decreasing in (0, π).

Q:  

32. For what values of a the function f given by f(x) = x2 + ax + 1 is increasing on [1, 2]?

A: 

We have, f (x) = x2 + x + 1

So, f (x) = 2x + a

If f (x) is strictly increasing onx  (1, 2), f (x)>0.

So, 1

2×1<2×x<2×2

2<2x<4

2+a<2x+a<4+a.

2+a<f (x)<4+a.

The minimum value of f (x) is 2 + a and that of men value is 4 + a.

∴ 2 +a> 0and 4 + a> 0

a> -2 and a> -4.

a> -2.

∴The best value of a is -2.

Q:  

15. The total cost C (x) in Rupees associated with the production of x units of an item is given by C (x) = 0.007x3 – 0.003x2 + 15x + 4000.

Find the marginal cost when 17 units are produced.

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A: 

Given, c (x) = 0.007 x3- 0.003x2 + 15x + 400.

Since the marginal cost is the rate of change of total cost wrt the output we have,

Marginal cost, MC, =dCdx (x)

= 0.007 * 3x2- 0.003 * 2x + 15.

When x = 17,

Then, MC = 0.007 * 2. (17)2 - 0.003 2 (17) + 15.

= 6.069 - 0.102 + 15.

= 20.967

Hence, the required marginal cost = ' 20, 97.

Q:  

33. Let I be any interval disjoint from [–1, 1]. Prove that the function f given by

f(x)=x+1x is increasing on I.

Read more
A: 

We have, f (x) = x? +1x

So, f (x) = 11x2=x21x2= (x1) (x+1)x2

So, for every x∈ I, where I is disjoint from [-1,1]

f (x) =  (+ve) (+ve) (+ve)= (+ve)>0whenx>1.

and f (x) =  (ve) (ve) (+ve) = ( + ve) > 0 when x< -1.

∴f (x) is strictly increasing on I .

Q:  

44. Find points at which the tangent to the curve y = x3 – 3x2 – 9x + 7 is parallel to the x-axis.

A: 

The given eqn of the curve is y=x33x29x+7.

slope of tangent to the given curve, dydx=3x26x9

when the tangent is parallel to x-axis dydx=0

3x26x9=0

x22x3=0

x2+x3x3=0

x(x+1)3(x+1)=0

(x+1)(x3)=0

 x = 3 or x = -1

When x = 3, y=333(3)29(3)+7=272727+7=20

And when x = -1 y=(1)33(1)29(1)+7=13+9+7=12

Hence, the required points are (3,20)(1,12)

Q:  

22. Find the intervals in which the function f given by f(x) = 2x2 – 3x is

(a) Increasing (b) Decreasing

Read more
A: 

We have, f (x) = 2x2 3x

So, f (x) = ddx (2x23x)=4x3.

Atf (x) = 0.

4x - 3 = 0

i e,  x=34 divides the real line into two

disjoint interval  (, 34) (34, )

(a) Now,

f (x) = 4x - 3 > 0 x (34, )

So, f (x) is strictly increasing in  (34, )

(b) Now, f (x) = 4x - 3 < 0 x (, 34)

So, f (x) is strictly decreasing in  (, 34)

Q:  

23. Find the intervals in which the function f given by f(x) = 2x3 – 3x2 – 36x + 7 is

(a) Increasing (b) Decreasing

Read more
A: 

We have, f (x) = 2x3- 3x2- 36x + 7.

So, f (x) = ddx (2x33x236x+7)=6x26x36.

= 6 (x2-x- 6).

= 6 (x2- 3x + 2x- 6)

= 6 [x (x- 3) + 2 (x- 3)]

= 6 (x- 3) (x + 2).

At, f (x) = 0

6 (x- 3) (x + 2) = 0.

So, when x- 3 = 0 or x + 2 = 0.

x = 3 or x = -2.

Hence we an divide the real line into three disjoint internal

(, 2) II (2, 3)andIII (3, )

At x ∈ (, 2),

f (x) = ( + ve) ( -ve) ( -ve) = ( + ve) > 0.

So, f (x) is strictly increasing in  (, 2)

At, x∈ ( -2,3),

f (x) = ( + ve) ( + ve) ( -ve) = ( -ve) < 0.

So, f (x) is strictly decreasing in ( -2,3).

At, x ∈ (3, )

f (x) = ( + ve) ( + ve) ( + ve) = ( + )ve> 0.

So, f (x) is strictly increasing in  (3, )

∴ (a) f (x) is strictly increasing in  (, 2)and (3, )

(b) f (x) is strictly decreasing in ( -2,3)

Q:  

19. Show that the function given by f (x) = 3x + 17 is increasing on R.

A: 

We have, .

f (x) = 3x + 17.

So, f (x) = 3 > 0 xR

∴f (x) is strictly increasing on R.

Q:  

20. Show that the function given by f (x) = e2x is increasing on R.

A: 

We have, f (x) = e2x

So, f (x) = ddxe2x = e2xddx2x = 2e2x> 0 xR.

∴f (x) is strictly increasing on R.

Q:  

25. Show that 2 log(1 ) 2 x y x x = +− + , x > – 1, is an increasing function of x throughout its domain.

Read more
A: 

We have, y = log (1+x)2x2+x,x>1

Differentiating the above wrt.x.we get,

dydx=11+xddx(1+x) (2+x)ddx2x2xddx(2+x)(2+x)2

dydx=11+x(2+x)22x(2+x)2.

dydx=11+x4+2x2x(2+x)2=11+x4(2+x)2.

=(2+x)24(1+x)(1+x)(2+x)2

=4+x2+4x44x(1+x)(2+x)2

dydx=x2(1+x)(2+x)2.

The given domain of the given function isx> -1.

 (x + 1) > 0.

Also, x20

(2 + x)2> 0.

Hence, dydx=(+ve)(+v)(+ve)=(+ve)>0.

∴ y is an increasing function of x throughout its domain.

Q:  

24. Find the intervals in which the following functions are strictly increasing or decreasing:

(a) x2 + 2x – 5 (b) 10 – 6x – 2x2 (c) –2x3 – 9x2 – 12x + 1 (d) 6 – 9x – x2

(e) (x + 1)3 (x – 3)3 

Read more
A: 

(a) f (x) = x2 + 2x - 5.

f(x) = 2x + 2 = 2 (x + 1).

At, f(x) = 0

2 (x + 1) = 0

x = -1.

At, x (,1),

f(x) = (- ) ve< 0.

So, f (x)is strictly decreasing or (,1).

At x ∈(1,)

f(x) = ( + ve) > 1

f(x) is strictly increasing on (1,).

(b) f(x) = 10 - 6x- 2x2

So, f(x) = - 6 - 4x = - 2 (3 + 2x).

Atf(x) = 0

 2 (3 + 2x) = 0.

 x = 32

At x (,32),

∴f(x) is strictly increasing on (,32)

At x (32,)

f(x) = ( -ve) ( + ve) = ( - ) ve< 0.

∴f (x) is strictly decreasing on (32,)

(c) f (x) = 2x3- 9x2- 12x.

So, f (x) =- 6x2- 18x- 12 = - 6 (x2 + 3x + 2).

= -6 [x2 + x + 2x + 2]

= -6 [x (x + 1) + 2 (x + 1)]

= -6 (x + 1) (x + 2)

At, f (x) = 0.

 6 (x + 1) (x + 2) = 0

x = -1 and x = -2.

At, x (,2) , f(x) = ( -ve) ( -ve) ( -ve) = ( -ve) < 0

So, f(x) is strictly decreasing.

At, x( -2, -1), f(x) = ( -ve) ( -ve) ( -ve) = ( -ve) < 0.

So, f(x) is strictly decreasing

(d) f(x) = 6 - 9x-x2

So, f(x) = - 9 - 2x

At, f(x) = 0

9 - 2x = 0

x =92

At, x ∈(,92),

f(x) > 0

So, f(x) is strictly increasing.

At x ∈(92,).

f(x) < 0.

So, f(x) is strictly decreasing.

(e) f(x) = (x + 1)3 (x- 3)3.

So, f(x) = (x + 1)3 ddx(x3)3+(x3)3ddx(x+1)3

= (x + 1)3. 3 (x- 3)2 + (x- 3)3. 3 (x + 1)2

= 3 (x + 1)2 (x- 3)2 (x + 1 + x- 3).

= 3 (x + 1)2 (x- 3)2 (2x- 2).

= 6 (x + 1)2 (x- 3)2 (x- 1).

For strictly increasing,

f(x) > 0.

As 6 > 0,(x + 1)2> 0 and (x- 3)2> 0. We need

(x 1) > 0.

 x> 1.

∴f (x) is strictly increasing on (1,).

And strictly decreasing on (,1).

Q:  

42. Find the slope of the normal to the curve x=acos3θ,y=asin3θ,at,θ=π4

A: 

The Equation of the given curve are

x=acos3θy=asin3θ

So,  dxdθ=3acos2θ (sinθ)=3acos2θsinθ.

and dydθ=3asin2θcosθ

dydx=dy/dθdx/dθ=3asin2θcosθ3acos2θsinθ=tanθ

So,  dydx|x=π/4=tanπ4=1 which is the slope of the tanget to the curve.

Now, required slope of normal to the curve =1
Slopeoftangent
 
=11=1

Q:  

92. Show that of all the rectangles inscribed in a given fixed circle, the square has the maximum area.

Read more
A: 

Let A.B.C.D.be the square increased in a given fixed circle with radius x

Let ‘x’ and ‘y’ be the length and breadth of the rectangle

∴x, y> 0

In ABC, right angle at B,

x2 + y2 = (2x)2

x2 + y2 = 4x2

Q:  

30. Which of the following functions are decreasing on (0,π2) ?

(A) cos x (B) cos 2x (C) cos 3x (D) tan x

Read more
A: 

(A) We have,

f(x) = cosx

So, f(x) = -sin x

When x [0,π2) we know that sin x> 0.

-sinx< 0. f(x) < 0 x(0,π2)

∴f(x) is strictly decreasing on (0,π2)

(B) We have, f(x) = cos 2x

So, f(x) = -2sin 2x

When x(0,π2) we know that sin x> 0.

i e, 0 π2
=> 0<2x<π 

So, sin 2x> 0 (sinØ is ( +ve) in 1st and 2ndquadrant).

-2sin 2x< 0.

 f (x) < 0.

∴f (x) is strictly decreasing on (0,π2).

(c) We have, f(x) = cos 3x

f(x) = -3 sin 3x.

As 0<x<π2

0<3x<3π2

We can divide the interval into two

Case I At, 0 < 3x

sin 3x> 0.

-3 sin 3x< 0 {?0<Bx<π

0<x<π3.

 f(x) < 0.

∴f(x) is strictly decreasing on (0'π3)

case II. At π<3x<3π2weget(iiirdquadrout).

sin 3x< 0.

-3 sin 3x> 0.

 f(x) > 0. {?π<3x<3π2π3<x<π2}

∴f(x) is strictly increasing on (π3,π2)

Hence, f(x) is with a increasing or decreasing on (0,π2).

(d) We have, f(x) = tan x.

So, f(x) = sec2x.

f(x) = sec2x> 0 x(0,π2).

∴f(x) is strictly increasing on (0,π2).

Hence, the fxncosx and cos 2x are strictly decreasing on (0,π2).

Q:  

97. Show that the right circular cone of least curved surface and given volume has an altitude equal to √2 time the radius of the base.

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A: 

Let r and h be the radius and height of the cone.

The volume V of the cone is.

=13πr2h

r2h=3Vπ=k (SAY)r2=kh.

And curve surface area S is

Q:  

1. Find the rate of change of the area of a circle with respect to its radius r when

ddr=ddrr2) = 2πr.

A: 

(a) r = 3cm

When r = 3 cm,

ddr = 2 × π × 3 cm = 6π.

Thus, the area of the circle is changing at the rate of 6π cm cm25.

(b) r = 4cm

whenr = 4cm,

ddr = 2 × π × 4cm = 8π.

Thus, the area of the circle is changing at the rate of 8 cm25.

Q:  

31. On which of the following intervals is the function f given by f(x) = x100 + sin x –1 decreasing ?

(A) (0,1) (B)  (π2,π) (C) (0,π2) (D) None of these

Read more
A: 

We have, f (x) = x 100 + sin x - 1.

So, f (x) = 100x99 + cosx.

(A) When x (0,1). We get.

x>0

x99> 0

100 x99> 0.

Now, 0 radian = 0 degree

and 1 radian = 180°×1π=180°(227)=57°.

So, cosx> 0 for x∈ (0,1) radian = (0,57)

∴f (x) > 0 for x∈ (0,1).

(B) When x ∈(π2,π) we get,

So, x> 1

x99> 1

100x99> 100. {?(π2,π)=(1.57,3.14) which is great than 1}

And cosx is negative between -1 and 0.

So, f (x) = 100x99 + cosx> 100 - 1 = 99 > 0.

∴f(x) is strictly increasing on (π2,π)

(c) When x ∈(0,π2), we get,

 x> 0

x99> 0

100x99> 0

and cosx> 0. (firstquadrant).

I e, f(x) > 0.

∴f(x) is strictly increasing on (0,π2)

Hence, option (D) is correct.

Q:  

Q38. Find the slope of the tangent to the curve y = 3x4 – 4x at x = 4.

A: 

The given eqn of the curve is y=3x44x.

Slope of the tangent at x = 4 is given by

dydx]x=4=12x34]x=4=12 (4)34=12×644

=7684

= 764

Q:  

62. Find the equation of the tangent to the curve y = √3x - 2 which is parallel to the line 4x - 2y + 5 = 0.

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A: 
Kindly go through the solution

Q:  

18. The total revenue in Rupees received from the sale of x units of a product is given by R(x) = 3x2 + 36x + 5. The marginal revenue, when x = 15 is

(A) 116 (B) 96 (C) 90 (D) 126

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A: 

Given, R (x) = 3x2 + 36x + 5.

Marginal revenue,  ddxR (x)=ddx (3x2+36x+5)

= 3 * 2x + 36

= 6x + 36

When x = 15.

ddxR (x)=6*15+36 = 90 + 36 = 126

Option (D) is correct.

Q:  

29. Prove that the function f given by f(x) = x2 – x + 1 is neither strictly increasing nor decreasing on (– 1, 1).

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A: 

We have, f (x) = x2-x + 1.

So, f (x) = ddx (x2x+1)=2x1

Atf (x) = 0.

2x - 1 = 0

I e, x = 12 divides the real line into two disjoint interval the interval ( -1, 1) into

Two disjoint interval

(1, 12) (12, 1)

And f (x) is strictly increasing in  (12, 1) and strictly decreasing in  (1, 12)

Hence, f (x) is with a increasing or decreasing on ( -1,1).

Q:  

11. A particle moves along the curve 6y = x3 +2. Find the points on the curve at which the y-coordinate is changing 8 times as fast as the x-coordinate

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A: 

Given eqn of the curve is 6y = x3 + 2.______ (1)

Wheny coordinate change s 8 times as fast as x-coordinate

dydx = 8 _____ (2)

Now, differentiating eqn (1) wrt.x we get,

ddx (6y)=ddx (x3+2)

6dydx=3x2+0.

6 × 8 = 3x2 (using eqn (2)

x2=483=16

+√x=±√16

x = ±4.

When x = 4, we have, 6y = 43+ 2 = 64 + 2 + 66

y=666 y =11.

And when x = -4, we have, 6y = ( -4)3 + 2 = -64 + 2 = -62

y=626=313

The tequired point s are (4, 11) and  (4, 313)

Q:  

13. A balloon, which always remains spherical, has a variable diameter 3 (2 1) 2 x + . Find the rate of change of its volume with respect to x.

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A: 

Given, diameter of the spherical balloon = 32 (2x + 1)

So, radius of the spherical r = 12×32(2x+1)

=34(2x+1)

Then, volume of the spherical V = 49πr3

=43π×[34(3(2x+1)]3

=9π16(2x+1)3.

Q Rate of change of volume wrt.tox, dVdx=ddx[π16(2x+1)3]

=9π16×3×(2x+1)2·ddx(2x+1)

=27π16(2x+1)2×2=27π8(2x+1)2.

Q:  

26. Find the values of x for which y = [x(x – 2)]2 is an increasing function.

A: 

We have, y = [x (x- 2)]2.

Differentiating the above w rt. x we get,

dydx=ddx[x](x2)2

=2[x(x2)]ddx[x(x2)]

=2[x(x2)][xddx(x2)+(x2)dxdx]

= 2 [x (x- 2)] (x + x- 2)

= 2x (x - 2) (2x - 2)

dydx = 4x (x - 2) (x - 1).

Now, dydx=0

4x (x - 2) (x - 1) = 0.

i e, x = 0, x = 2, x = 1 divides the real line into

four disjoint interval. (,0], [0,1],[1,2] and [2,].

when x [,0].

dydx=(ve)(ve)(ve)= (ve)0,0x=0

∴f (x) is decreasing in [,0].

When x[0,1]

dydx=(+ve)(ve)(ve) =(+ve)0,x=0x=1

∴f (x) is increasing in [0,1].

When x [1,2]

dydx=(+ve)(ve)(+ve)=(ve)0, 0for x=1+x=2

∴f (x) is decreasing.

When x(2,).

dydx=(+ve)(+ve)(+ve)=(+ve)0,0,for x=2

∴f (x) is in creasing

Hence, f (x) is increasing for x [0,1)x[2,)

Q:  

28. Prove that the logarithmic function is increasing on (0, ∞).

A: 

We have, f (x) = log x.

So, f (x) = ddx (logx)=1x.

i e, f (x) =1x > 0. When x0x (0, )

Hence, the logarithmic fx is strictly increasing on (0, ).

Q:  

34. Prove that the function f given by f(x) = log sin x is increasing on (0,π2) and decreasing on (π2,π) .

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A: 

We have, f (x) = log sin x

So, f (x) = 1sinxdsinxdx=cosxsinx=cotx

When x (0, π2)

f (x) = cot x> 0 (Ist quadrat )

So, f (x) is strictly increasing on  (0, π2)

When x ∈ (π2, π)

f (x) = cot x< 0. (IIndquadrant ).

So, f (x) is strictly decreasing on  (π2, π)

Q:  

35. Prove that the function f given by f (x) = log |cos x| is decreasing on (0,π2) and increasing on (3π2,2π)

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A: 

We have, f (x) = log |cosx|.

f (x) = 1cosxddxx=sinxcosx=tanx

whenx  (0, π2),  we get.

tanx> 0 (Ist quadrant).

 tanx< 0

 f (x) < 0.

∴f (x) is decreasing on  (0, π2).

When x ∈ (3π2, 2π) we get,

tanx|< |0 (ivth quadrant).

-tanx|>| 0

 f (x) > 0

∴f (x) is increasing on  (3π2, 2π).

Q:  

37. The interval in which y = x2 e–x is increasing is (A) (– ∞, ∞) (B) (– 2, 0) (C) (2, ∞) (D) (0, 2)

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A: 

We have, f (x) = x2 e–x

So, f (x) = x2ddxex+exdx2dx

=x2exddx (x)+ex2xdxdx

= -x2 e-x + e-x 2x.

= x e-x ( x + 2).

=x (2x)eex

If f (x) = 0.

x (2x)ex=0.

 x = 0, x = 2.

Hence, we get there disjoint interval

[, 0), (0, 2) (2, )

When, x  (, 0), we have, f (x) = ( -ve) ( + ve) = ( -ve) < 0.

So, f is strictly decreasing.

When x ∈ (0,2), f (x) = ( + ve) ( + ve) = ( + ve) > 0.

So, f is strictly increasing.

And when x ∈ (2, ), f (x) = ( +ve) ( -ve) = ( -ve) < 0.

So, f is strictly decreasing.

Hence, option (D) is correct.

Q:  

66. Find the approximate value of f(2.01), where f (x) = 4x2 + 5x + 2.

A: 

Given, y = f (x) = 4x2 + 5x + 2.

So, f (x) = 8x + 5. dydx = 8x + 5 dy = (8x + 5) dx.

Let x = 2 and Δx = 0.01.Then,

f (x + Δx) = f (2 + 0.01) = f (2.01).

Δy = f. (x + Δx) f (Δx).

f (x +Δx) = f (x) +Δy.

= f (x) + dy = f (x) + (8x + 5) dx.

= f (2.01) = f (2) + (8 x 2 + 5). Δx {∴dx = Δx}

= 4 (2)2 + 5 (2) + 2 + 21 (0.01)

= 16 + 10 + 2 + 0.21 = 28.21.

Q:  

68. Find the approximate change in the volume V of a cube of side x metres caused by increasing the side by 1%.

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A: 

We know that, the volume v of side ‘a’ mete of cube is v = x3.

So,  d= (ddx)Δx=3x2Δx.

Given that, increase in side = 1% of x.

Δx=x100

dv=3x2 (x100)=0.03x3 m3.

Q:  

82. What is the maximum value of the function sin x + cos x?

A: 

We have, f (x) = sin x + cos x.

f (x) = cos x - sin x.

At f (x) = 0

cosx - sin x = 0

sinx = cos x

sinxcosx=1.

tanx=1=tanπ4

x=π4orx=nπ+π4.

At x=π4+nπ ,

f (nπ+π4)=sin (nπ+π4)+cos (nx+π4).

= (1)xsinπ4+ (1)nsinπ4.

Q:  

85. Find the maximum and minimum values of x + sin 2x on [0, 2π].

A: 

We have, f(x) = x + sin 2x ,x ∈ [0, 2π].

f(x) = 1 + 2cos 2x

At f(x) = 0

1 + 2 cos2x = 0

cos2x=12=cosπ3=cosππ3=cos2π3

2x=2nπ±2π3,n=1,2,3.

x=nπ±2π3

n=0,x=±π3x=π3[0,2π].

n=1,x=π±π3x=π+π3andππ3

=4π3ad2π3[0,2π]

n=2,x=2π±π3x=2π+π3ad2ππ3.

x=513[0,2π].

Hence, x=π3,2π3,4π3and5π3

Missing

At x=π3,f(π3)=π3+sin2π3=1.05+ sin(ππ3)=1.05+sinπ3

=1.05+√3/2

= 1.05 + 0.87

= 1.92

At x=2π3,f(2π3)=2π3+sin2×2π3 =2.10+sin(π+π3)

=2.10sinπ3=2,100.87

= 1.23

At x=4π3,f(4π3)=4π3+sin2×4π3=42+sin(3xπ3)

=4.2+sinπ3=4.2+0.87.

=5.07.

At x=5π3,f(5π3)=5π3+sin2×5π3=5.25+sin(3x+π3)

=525sin13

= 5.25 - 0.87 = 4.38

At and points,

f(0) = 0 + sin2 × 0 = 0

f(2π) = 2π + sin 2 × 2π = 6.2 + 0 = 6.28

∴Maximum value of f(x) = 6.28 at x = 2π and

minimum value of f(x) = 0 at x= 0

Q:  

120. Show that height of the cylinder of greatest volume which can be inscribed in a right circular cone of height h and semi vertical angle α is one-third that of the cone and the greatest volume of cylinder is 427πh3tan2α .

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A: 

Kindly go through the solution

Q:  

123. The normal at the point (1,1) on the curve 2y + x2 = 3 is (A) x + y = 0 (B) x – y = 0 (C) x + y +1 = 0 (D) x – y = 1

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A: 

The equation of the given curve is 2y+x2=3 .

Differentiate with respect to x, we have:

2dydx+2x=0dydx=xdydx] (1, 1)=1

The slope of the normal to the given curve at point (1,1) is

1dydx] (1, 1)=1

Hence, the equation of the normal to the given curve at (1,1) is given as:

y1=1 (x1)y1=x1xy=0

Therefore, option (B) is correct.

Q:  

2. The volume of a cube is increasing at the rate of 8 cm3 /s. How fast is the surface area increasing when the length of an edge is 12 cm?

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A: 

Let x be the length of edge,v be the value and s be the surface area of the cube then,

y = x3.

and S = 6x2, where x is a fxn of time.

Now, dYdF=8cm35

ddt (x3) = 8

dx3dx·dxdt=8 (by chain rule)

 3x2 dxdx=8.

dxdt=83x2.

Now, dSdt=ddt (bx2) = d(6x2)dxdxdx = 12x 83x2=32x cm25.

When x = 12 cm,

dSdt=3212=83 cm25.

Q:  

3. The radius of a circle is increasing uniformly at the rate of 3 cm/s. Find the rate at which the area of the circle is increasing when the radius is 10 cm.

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A: 

Let ‘r’ cm be the radius of the circle which is afxn of time.

Then,  dydt = 8 3cm/s as it is increasing.

Now, the area A of the circle is A = πr2.

So, the rate at which the area of the circle change ddt=ddt πr2.

=ddrπr2·drdt

= 2πr 3

= 6πr. cm25

When r = 10cm,

ddt = 6.π × 10 = 60π cm25

Q:  

4. An edge of a variable cube is increasing at the rate of 3 cm/s. How fast is the volume of the cube increasing when the edge is 10 cm long?

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A: 

Let 'x' cm be the length of edge of the cube which is a fxn of time t then,

dxdt = 3cm/s as it is increasing.

Now, volume v of the cube is v = x3

Ø Rate of change of volume of the cube dvdt=dq3dt.

=dx3dxdxdt

= 3x2.3

= 9x2 cm25

When x = 10cm.

dvdt = 9 x (10)2= 900 cm25

Q:  

5. A stone is dropped into a quiet lake and waves move in circles at the speed of 5 cm/s. At the instant when the radius of the circular wave is 8 cm, how fast is the enclosed area increasing?

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A: 

The area A of the circle with radius π is A = πr2

Then, rate of change in area dAdt=dx? r2dt = dxr2dr·drdt

= 2πr dr
dt.

Q The wave moves at a rate 5cm/s we have,

drdF = 5cm/s

So,  dAdt =r. 5 = 10πr. cm25

When r = 8 cm.

ddt = 10.π.8 cm cm25 = 80 × cm cm25

Q:  

6. The radius of a circle is increasing at the rate of 0.7 cm/s. What is the rate of increase of its circumference?

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A: 

The circumference C of the circle with radius r is C = 2πr.

Then, rate of change in circumference is dCdt=ddt2πr = 2πdxdf.

Q Radius of circle increases at rate 0.7 cm/s we get,

dydt=0.7 cm/s

So,  dCdt = 2.× 0.7 cm/s = 1.4 × cm/s.

Q:  

7. The length x of a rectangle is decreasing at the rate of 5 cm/minute and the width y is increasing at the rate of 4 cm/minute. When x = 8cm and y = 6cm, find the rates of change of (a) the perimeter, and (b) the area of the rectangle.

Read more
A: 

Since the length x is decreasing and the widthy is increasing with respect to time we have

dxdt = 5cm/min and dydt = A cm/min

(a) The perimeter P of a rectangle will be, P = 2 (x + y)

Q  Rate of change of perimeter, dPdt=d2dt (x+y)

=2 (dxdt+dydt)

= 2 ( -5 + 4)

= -2 cm/min

(b) The area A of the rectangle is A = x. y.

Rate of change of area is dAdt=ddt (x·y)

=xdydt+dxdt·y.

= 4x - 5y

So,  dAdt |x = 8ay = 6cm = 4 (8) - 5 (6) = 32 - 30 = 2 cm2/min spherical balloon

Q:  

8. A balloon, which always remains spherical on inflation, is being inflated by pumping in 900 cubic centimetres of gas per second. Find the rate at which the radius of the balloon increases when the radius is 15 cm

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A: 

Let ‘r’ cm be the radius of volume V. measured Then,

V =43πr3

Now, rate at which balloon is being inflated = 900cm35

dvdr=900  cm35

ddt(43πr3) = 900 cm35

ddr(43×r3)drdt=900

43π × 3 × r2 drdr = 900.

drdt=9004πr2.

When r = 15cm,

dsdt|r=15=9004π×(15)2 = 900900π=1π cm/s.

Q Radius of balloon increases by 1π per second.

Q:  

9. A balloon, which always remains spherical has a variable radius. Find the rate at which its volume is increasing with the radius when the later is 10 cm

Read more
A: 

The volume v of a spherical balloon with radius r is V.43πr3.

with respect its radius.

Then, the rate of change of volume |dVdr=ddr (43·πr3)

=43π (ddrr3)

=43π×3×π2

= 4π r2

Whenx = 10 cm,

dVdr = 4 π10)2 = 400 πcm3/cm

Q:  

10. A ladder 5 m long is leaning against a wall. The bottom of the ladder is pulled along the ground, away from the wall, at the rate of 2cm/s. How fast is its height on the wall decreasing when the foot of the ladder is 4 m away from the wall ?

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A: 

Since, the bottom of ground is increasing with time t,

dxdt = 2cm/s

From fig, Δ ABC, by Pythagorastheorem

AB2 + BC2 = AC2

x2 + y2 = 52

x2 + y2 = 25 ____ (1)

Differentiating eqn (1) w. r. t. time t we get,

ddt(x2+y2)=ddt(25)

2xdxdt+2ydydt=0.

2x×2+2y2ydy=0

dydt=2xy m/s

When x = 4m, the rate at which its height on the wall decreases is

dydt=2×43 {42+y2=52y2=2516y √9(L engthcan'tbenegetive)y=3

dydt=83 room

Q:  

12. The radius of an air bubble is increasing at the rate of 1 2 cm/s. At what rate is the volume of the bubble increasing when the radius is 1 cm?

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A: 

Let x be the radius of the bubble with volume .V. then,

drdt=12 cm/s

andV = 43πn3

Rate of change of volume dVdt=ddt (43π3) = 43πddt4r3

=43π×3r2drdt.

= 4πr2 ×12

= 2πr2.

dvdt|r=cm = 2x (1)2 2π. cm35

Q:  

16. The total revenue in Rupees received from the sale of x units of a product is given by R (x) = 13x2 + 26x + 15. Find the marginal revenue when x = 7.

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A: 

Given, R (x) = 13x2 + 26x + 15.

Marginal revenue is the rate of change of total revenue with respect to the number of units sold Marginal revenue (MR) = dR (x)dx

=dd (13x2+26x+15)

= 13 * 2x + 26

= 26x + 26

When x = 7,

MR = 26 * 7 + 26 = 182 + 26 = 208.

Hence, the required marginal reverse = ' 208.

Choose the correct answer for questions 17 and 18.

Q:  

17. The rate of change of the area of a circle with respect to its radius r at r = 6 cm is

(A) 10π (B) 12π (C) (D) 11π

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A: 

The area A of the isle with radius r is given by with respect to radius r A = πr2.

Then, rate of change of area of the circle d·Adx=dπr2dr

= 2πr.

When r = 6 cm

dAdt=2π×6=12π.

Q option (B) is correct.

Q:  

39. Find the slope of the tangent to the curve y=x1x2,x2,at,x=10

A: 

The given eqn of the curve is y=x1x2

Slope of tangent at x = 10 is given by,

dydx|x=10=(x2)ddx(x1)(x1)ddx(x2)(x2)2|x=10

=(x2)(x1)(x2)2|x=10=x2x+1(x2)2|x=10

=1(x2)2|x=10

=1(102)2=182=164

Q:  

40. Find the slope of the tangent to curve y = x3 – x + 1 at the point whose x- coordinate is 2.

A: 

Slope of tangent to the given curve y=x3x+1 is

dydx=3x21.

So,  dydx|x=2=3 (2)21=121=11.

Q:  

41. Find the slope of the tangent to the curve y = x3 –3x + 2 at the point whose x-coordinate is 3.

A: 

Slope of tangent to the given curve y=x33x+2 is dydx=3x23.

so,  dydx|x=3=3 (3)23=273=24.

Q:  

43. Find the slope of the normal to the curve x=1sinθ,y=bcos2θ,at,θ=π2

A: 

The given eqn of the curves are

x=1asinθy=bcos2θ

so,  dxdθ=acosθdydθ=2bcosθsinθ

dydx=dy/dθdx/dθ=2bcosθsinθacosθ=2basinθ

Slope of tangent to curve at θ=π2 is dydx|θ=π2

=2basinπ2

=2ba

Hence, slope of normal to curve =12b/a=a2b

Q:  

45. Find a point on the curve y = (x – 2)2 at which the tangent is parallel to the chord joining the points (2, 0) and (4, 4)

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A: 

Let the point joining the chord be  (2, 0) (4, 4)

Then slope of the chord =4042 {? Slope=y2y1x2x1}

=42

= 2

The given eqn of the curve y= (x2)2 

slope of the tangent to the curve dydx=2 (x2).

Given that, the tangent is parallel to the chord PQ.

slope of tangent = slope of PQ.

2 (x2)=2.

x=1+2

x=3.

and y= (32)2=12=1.

The required point on curve is  (3, 1)

Q:  

46. Find the point on the curve y = x3 – 11x + 5 at which the tangent is y = x – 11.

A: 

The given eqn of the curve is y=x311x+5

slope of tangent to the curve dydx=3x211

Then eqn of tangent is y=x11 xy11=0 which gives us slope =11=1

So, 3x211=1

3x2=1+11=12

x2=4

x=±2

When x = 2, y=2311(2)+5=822+5=9.

And when x = 2, y=(2)311(2)+5=8+22+5=19.

The point (2,9) when put into y=x11. we get

9=211

9=9 which is true.

and the point (2,19) when put into y=x11 gives,

19=211

19=13 which is not true.

Hence, the required point is (2,9) 

Q:  

47. Find the equation of all lines having slope – 1 that are tangents to the curve y=1x1,x1

A: 

The given eqn of curve is y=1x1

Slope of tangent to the given curve is dydx=1(x1)2

Given that, slope of tangent = 1.

1(x1)2=1.

(x1)2=1.

x1=±1

x=1±1.

ie, X=1+1 or x=11

x=2 or x=0

When x=2,y=121=1

and when x=0,y=101=1.

Hence, the point of contact of the tangents are (2,1)and(0,1)

The reqd. eqn of line are y1=(1)(x2){?yy0=m(xx0) eqn of line 

and y(1)=(1)(x0).

y1=x+2 and y+1=x

x+y3=0 and x+y+1=0.

Q:  

48. Find the equation of all lines having slope 2 which are tangents to the curve

y=1x3,x3

A: 

The given eqn of curve is y=1x3

Slope of tangent to the curve is dydx=1 (x3)2.

Given,  dydx=2

1 (x3)2=2

(x3)2=12 which is not possible

we conclude that there is no possible tangent to the given curve with slope = 2.

Q:  

49. Find the equations of all lines having slope 0 which are tangent to the curve

y=1x22x+3

A: 

The given eqn of the curve is y=1x22x+3

Slope of tangent to the curve is dydx=1(x22x+3)=ddx(x22x+3)

=(2x12)(x22x+3)2

Given, dydx=0

(2x2)(x22x+3)2=0

2(x1)=0

x=1

When x=1,y=1122×1+3=112+3=12

The point of contact of the tangent to the curve is (1,12)

The eqn of the line is y12=0(x1)

y=12

Q:  

50. Find points on the curve x29+y216=1 at which the tangents are

(i) Parallel to x-axis (ii) Parallel to y-axis.

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A: 

Diffrentiating x29+y216=1. wrt. X we get,

2x9+2y16dydx=0

2ydy16dx=2x9

dydx=16x9y

(i) When the tangent is || to x-axis, the slope of tangent is 0

ie, dydx=0

16x9y=0

x=0 putting this in the eqn of curve. We get,

029+y216=1

y2=16

y=±4.

The point at which the tangents are parallel to x-axis are (0,4)and (0,4)

(ii) When the tangent is parallel to y-axis, the slope of the normal is 0.

ie, 1dydx=0

dxdy=0

9y16x=0

y=0 , putting this in the eqn of curve we get,

x29+y216=1.

x2=9

x=±3

The point at which the tangents are parallel to y-axis are (3,0)and (3,0)

Q:  

51. Find the equations of the tangent and normal to the given curves at the indicated points:

(i)y=x46x3+13x210x+5,at,(0,5)(ii)y=x46x3+13x210x+5,at,(1,3)(iii)y=x3,at,(1,1)(iv)y=x2,at,(0,0)(v)x=cosy,y=sint,at,t=π4

A: 

(i) we have, y=x46x3+13x210x+5

slope of tangent, dydx=4x318x2+26x10

dydx|(x,y)=(0,5)=10.

slope of normal =110=110

Hence eqn of tangent at (0, 5) is

y5=10(x0)10x+y5=0

And eqn of normal at (0, 5) is

y5=110(x0)

10y50=x

x10y+50=0

(ii) We have, y=x46x3+13x210x+5

Slope of tangent, dydx=4x318x2+26x10.

dydx|(x,y)=(1,3)=4(1)318(1)2+26(1)10

=418+2610

= 30 28

= 2

Slope of normal =12

Hence eqn of tangent at (1, 3) is

y3=2(x1)

y3=2x2

2xy+1=0

And eqn of normal at (1,3) is

(y3)=12(x1)

2y6=x+1

x+2y7=0

(iii) We have, y=x3

Slope of tangent, dydx=3x2

dydx|(1,1)=3(1)2=3.

And slope of normal =13

Hence, eqn of tangent at (1, 1) is

y1=3(x1)

y1=3x3

3xy2=0

And eqn of normal at (1,1) is

y1=13(x1)

3y3=x+1

x+3y4=0.

(iv) We have, y=x2

Slope of tangent dydx=2x

dydx|(0,0)=0.

So, eqn of the tangent at (0,0) is

(y0)=0(x0)

y=0.

ie, x- axis

Hence, the eqn of normal is x = 0 ie, y-axis

(v) We have, x=costy=sint

dxdt=sint·dydt=cost

So, slope of tangent dydx=dy/dtdx/dt=costsint=cott

dydx|t=π4=cotπ4=1.

And slope of normal =11=1

Q:  

52. Find the equation of the tangent line to the curve y =  x2 – 2x +7 which is

(a) Parallel to the line 2x – y + 9 = 0 (b) Perpendicular to the line 5y – 15x = 13

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A: 

The eqnof the given curve is y=x22x+7

Slope of tangent, dydx=2x2

(a) The line 2xy+9=0y=2x+9 compared to y=mx+c gives,

Slope of line = 2.

If the tangent of the curve is parallel to the line

dydx=slope of line

2x2=2

x=42x=2

When x=2,y=(2)22(2)+7=7

Hence, the point of contact of the tangent is (2, 7)

The eqn of tangent is y7=2(x2)

y7=2x4

2xy+3=0

(b) The line 5y15x=13y=155x+135y=3x+135

compared to y=mx+c gives

slope of line = 3

As the tangent to the curve is ⊥ to the line.

dydx= -1/slope opf line

2x2=13

6x6=1

6x=5

x=56

When x=56 we get y=(56)22×56+7

=253653+7

=2560+25236=21736

Hence, the point of contact of the tangent is (56,21736)

And eqn of the tangent is

y21736=13(x56)

3y21712=x+56

x+3y2171256=0

x+3y2171012=0

x+3y22712=0

12x+36y227=0.

Q:  

53. Show that the tangents to the curve y = 7x3 + 11 at the points where x = 2 and x = – 2 are parallel.

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A: 

The given eqn of the curve is y=7x3+11 .

Slope of tangent dydx=21x2

dydx|x=2=21 (2)2=21×4=84.

and dydx|x=2=21 (2)2=21×4=84

The tangent to the given curve at x = 2 and x = -2 are parallel.

Q:  

54. Find the points on the curve y = x3 at which the slope of the tangent is equal to the y-coordinate of the point.

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A: 

The given eqn of the curve is y=x3 .

Slope of tangent,  dydx=3x2

As, slope of tangent = y – coordinate of the point.

dydx=y

3x2=x3

3x2x3=0

x2 (3x)=0

x=0x=3

When x=0, y=0

and when x=3·, y=33=27.

The required points are  (0, 0)and (3, 27)

Q:  

55. For the curve y = 4x3 – 2x5 , find all the points at which the tangent passes through the origin.

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A: 

The given eqn of the curve is y=4x32x5

Slope of tangent, dydx=12x210x4. ________(1)

Let P(x, y) be the required point at the tangent passing through the origin (0,0)

Then, dydx=y0x0=yx _________(2)

So, from (1) and (2) we get,

yx=12x210x4.

y=12x310x5.

Putting this value of y in the eqn of curve we get,

12x310x5=4x32x5.

8x38x5=0

8x3(1x2)=0

x3=0 or 1x2=0

x=0 or x=±1.

When, x=0,y=4(0)32(0)5=0

x=1,y=4(1)32(1)5=42=2

x=1,y=4(1)32(1)5=4+2=2

The required points are (0,0), (1,2) and (1,2)

Q:  

56. Find the points on the curve x2 + y2 – 2x – 3 = 0 at which the tangents are parallel to the x-axis.

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A: 

The given eqnof the curve is x2+y22x3=0 ________ (1)

Differentiating the given curve wrt.x we get,

2x+2ydydx2=0.

2ydydx=22x

dydx=1xy slope of tangent

Given, tangent is | to x-axis

ie,  dydx=0

1xy=0

x=1

Putting x = 1 in eqn (1) we get,

12+y22 (1i)3=0

y24=0

y2=4

y=±2

Hence, the required points are (1,2) and (1, 2).

Q:  

57. Find the equation of the normal at the point (am2 ,am3 ) for the curve ay2 = x3

A: 

The given eqn of curve is ay2=x3 _____(1)

Differentiating eqn (1) wrt.x we get,

2aydydx=3x2.

dydx=3x22ay , slope of tangent

dydx|(x,y)=(am2,am3)=3[am2]22a[am3]=3a2m42a2m3=3m2

corresponding slope of normal =1(3m/2)=23m

Hence, eqn of normal at (am2,am3) is

yam3=23m(xam2).

3my3am4=2x+2am2

2x+3my3am42am2=0

2x+3myam2(2+3m2)=0

Q:  

58. Find the equation of the normals to the curve y = x3 + 2x + 6 which are parallel to the line x + 14y + 4 = 0.

Read more
A: 

The given eqn of the curve is y=x3+2x+6 _____(1)

slope of tangent to the curve, dydx=3x2+2

so, slope of normal to the curve =13x2+2

Now, the line x+14y+4=0y=114x414 compared to y=mx+c gives

slope of line = 114

As the normal is parallel to the line

13x2+2=114

3x2+2=14

3x2=12

x2=4

x=±2

When x=2,y=(2)3+2(2)+6=8+4+6=18

and when x=2,y=(2)3+2(2)+6=84+6=6

The point of contact of the normal are (2, 18) and (-2, -6)

Hence the eqn of normal are

y18=114(x2) and y(6)=114[x(2)]

14y252=x+2 y+6=114(x+2)

x+14y254=0 14y+84=x2

x+14y+86=0.

Q:  

59. Find the equations of the tangent and normal to the parabola y2 = 4ax at the point (at2 , 2at).

A: 

The given of the parabola is y2=4ax

slope of tangent is given by

ddxy2=ddx4ax

2ydydx=4a

dydx=2ay.

dydx|(at2,2at)=2a2at=1t

so, slope of normal

so, slope of normal =1(1t)=t

Hence eqn of tangent at point (at2,2at) is

y2at=1t(xat2)

ty2at2=xat2

xtyat2+2at2=0

xty+at2=0

And eqn of normal at point (at2,2at) is

(y2at)=(t)(xat2)

y2at=tx+at3

tx+y2at+at3=0

tx+yat(2+t2)=0

Q:  

60. Prove that the curves x = y2 and xy = k cut at right angles* if 8k2 = 1.

A: 

The given eqn of the curves are

x=y2 ________(1)

and xy=x ___________(2)

Differentiating eqn (1) and (2) wrt ‘x’ we get,

 dy2dx=dxdx

2ydydx=1

dydx=12y _________(3)

and ddx(xy)=ddx(x)

xdydx+y=0

dydx=yx _________(4)

Since the two curves cut each other at right angles we get,

(12y)×(yx)=1.

2x=1 ________(5)

The point of intersection can be solve from eqn (1) and (2), 

y2.y=k

y3=k

y=k1/3

x=y2=r2/3.

Hence, using eqn (5) we get,

2(r)2/3=1.

[2r2/3]3=13

8r2=1

Hence proved

Q:  

61. Find the equations of the tangent and normal to the hyperbola

x2a2y2b2=1 at the point (x0,y0)

A: 

The given eqn of the hyperbola is x2a2y2b2=1 ______(1)

Differentiating eqn (1) wrt ‘x’ we get,

2xa22yb2dydx=0

2yb2dydx=2xa2

dydx=b2xa2y

dydx|(x,y)=(x0,y0)b2x0a2y0 is the reqd slope of tangent to the curve

So, eqn of tangent at point (x0,y0) is

yy0=b2x0a2y0(xx0).

yy0b2y02b2=xx0a2x02a2

xx0a2yy0b2=x02a2y02b2

As (x0,y0) lies on the parabola given by eqn (1) we write,

x02a2y02b2=1

Hence, xx0a2yy0b2=1

Q:  

64. The line y = x + 1 is a tangent to the curve  y2 = 4x at the point (A) (1, 2) (B) (2, 1) (C) (1, – 2) (D) (– 1, 2)

Read more
A: 

The given eqn of curve is y2=4x .

Then, differentiating wrt x we get,

2ydydx=4

dydx=2y which is the slope of the tangent to the curve.

The line y=x+1 compared to y=mx+c gives slope of line = 1.

Since, tangent is the line we have,

2y=1.

y=2

Putting y = 2 in y2=4x we get,  4x=22x=1.

Hence, the required point is  (1, 2)

Option (A) is correct.

Q:  

65. Using differentials, find the approximate value of each of the following up to 3 places of decimal.

Read more
A: 

(i) Let y= ?x : Let x = 25 and x = 0. 3.

Then, ?y = ?x+?x??x

=?25.3??25

=?25.3?5

??25.3=5+?y?????????????????y~dy.

= 5 + dy

5+(dydx)Ax.

=5+12?x?x.

=5+12?25?0.3

=5+0.32×5

= 5 + 0.03

??25.3=5.030

(ii) ?49.5

A.(ii)

Let y = ?x. Let x = 49 and x = 0.5.

Then, ?y=?x+?x??x.

=?49.5??49

??49.5=7+?y=7+(dydx)?x

=7+12?x?x

=7+12?49×(0.5)

=7+0.514

= 7 + 0.0357.

??49.5=7?035

(iii) ?0.6

A.(iii)

Let y = ?x. Let x = 0.64 and ?x = 0.04.

Then, ?y=?x+Ax??x

=?0.64-0.04??0.64.

=?0.6?0.8

??0.6=0.8+?y=0.8+(dydx)?x

=0.8+12?x?x

=0.8+12?0.64×(??0.04)

=0.8?0.042×0.8

= 0.8 - 0.025.

= 0.775.

(iv) (0.009)13

A.(iv)

Let y=x13 Let x = 0.008 and ?x = 0.00 1.

Then, ?y = [x+?x]13?[x]13

=(0.009)13?(0.008)13

(0.009)13=0.2+?y=0.2+dydx??x

=0.2+13(x13)2?x

=0.2+13[(0.008)13]2×(0.001)

=0.2+0.0013×(0.0)2=0.2+0.0010.12

= 0.2 + 0.0083.

= 0.208.

(v) (0.999)110

A.(v)

Let y=x110. Let x = 1 and ?x = -0.001

Then, ?y=(x+?x)110?x110

=(0.999)110?(1)110

?1(0.999)110=1+Ay

=1+dydx?x.

=1+110x910×?x

=1?0.00110(.1910)=1?0.00110=1?0.0001

= 0.999.

(vi) (15)14

A.(vi)

Let y=x14. Then, x = 16 and ?x = 1.

Then, ?y=(x+Ax)14?x14.

=(15)14?(16)14

?(15)14=2+Ay=2+dydxAx

=2+14x34?(?1)

=4?14(16)34

=2?14×8

=4?132

=64?132=6332=1.968

(vii) (26)13

A.(vii)

Let y=x13. Let x = 27 and ?x = 1.

Then, ?y=(x+?x)13?x13

=(26)13?(27)13

?(26)13=3+4y=3+dydxAx=3+13x23Ax,

=3?13(2F)23

=3?127

=81?127=8027=2.962.

(viii) (255)14

A.(viii)

Let y=x14. Let x = 256 and ?x = 1.

Then, ?y=(x+?x)14?x14.

=(255)14?(256)14

?(255)14=(256)14+4y=4+dydxAx.

=4+14x34?4x

=4?14(256)34

=4?1256

=1024?1256

=1023256=3.996

(ix) (82)14

A.(ix)

Let y=x14. let x = 81 and ?x = 1.

Then, ?y=(x+4x)14?x14

=(82)14?(81)14

7(82)14=3+4y=3+dydx?x

=3+14x34??x

=3+14(81)34

=3+14×33

=3+1108=324+1108=325108=3.001.

(x) (401)12

A.(x)

Let y = x12 = ?x. Let x = 400 and ?x = 1.

Then, ?y=(x+?x)12?x12.

???y=(401)12?(400)12

??(401)12=20?+?y=20+dydx?x

=20+12?x?x

=20+12?400

=20+12×20

=20+140

=800+140=80140=20.025.

(xi) (0.0037)12

A.(xi)

Let y=x12 Let x = 0.0036 and ?x = 0.0001

Then, ?y=?x+?x??x

=?0.0037??0.0036

??0.0037=0.06+Dy=0.06+dydx?x=0.06+x2?x

=0.06+0.0012?0.0036

=0.06+0.0012×0.06

=0.06+0.0010.12.

= 0.06 + 0.0008.

= 0.0608.

(xii) (26.57)13

A.(xii)

Let y=x13. Let x = 27 and ?x = 0.43.

Then, ?y=(x+Ax)13?(x)13

=(26.57)13?(27)13

?(26.57)13=3+4y=3+dydx?x=3+13x23??x.

=3+13(27)23?
Q:  

67. Find the approximate value of f (5.001), where f(x) = x3 – 7x2 + 15.

A: 

Given, y = f (x) = x3- 7x2 + 15.

So,  dydx=f (x)=3x214x.

dy = (3x2- 14x) dx.

Δy = (3x2- 14x) Δx.

Let, x = 5 and Δx = 0.001. Then,

Δy = f (x + Δx) f (x).

f (x + Δx) = f (x) + Δy = f (x) + (3x2- 4x) Δx.

f (5 + 0.001) = 53- 7 (5)2 + 15 + [3 (5)2 - 14 (5)]. (0.001).

f (5.001) = 125 - 175 + 15 + (75 - 70) (0.001)

= -35 + 0.005 = - 34.995.

Q:  

69. Find the approximate change in the surface area of a cube of side x metres caused by decreasing the side by 1%

Read more
A: 

We know that, the surface area 5 of a ‘x’ when length cube, is S = 6x2.

So,  dS=dSdxΔx=12xΔx.

Given decrease in side,  Δx=1%x=x100.

dS=12x (x100)=0.12x2 m2.

Q:  

70. If the radius of a sphere is measured as 7 m with an error of 0.02 m, then find the approximate error in calculating its volume.

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A: 

Let x be the radius of the sphere & Δπ be the error in measuring the radius.

Then, π = 7m and Δr = 0.02m.

Now, volume v of sphere is

V=43πr3.

So,  dUdx=4πr2

dv= (dvdr)Δr=4πr2 (Δπ)

dv = 4π (7)2 (.0.02) = 3.92 πm3

∴The appropriate error is calculating the volume is 3.92πm3.

Q:  

71. If the radius of a sphere is measured as 9 m with an error of 0.03 m, then find the approximate error in calculating its surface area.

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A: 

Let be the radius of the sphere &r be the error in measuring the radius.

Then, π = 9m and Δr = 0.03m.

Now, surface area S of the sphere is

S = 4πr2

So,  dsdr=8πr.

∴e, this =  (dsdr) Δr = 8πr.Δr = 8π × 9 × 0.03

= 2.16πm3.

Appropriate error in calculating the surface area is 2.16πm3.

Q:  

72. If f(x) = 3x2 + 15x + 5, then the approximate value of f (3.02) is (A) 47.66 (B) 57.66 (C) 67.66 (D) 77.66

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A: 

We have, y = f (x) = 3x2 + 15x + 5.

dydx=6x+15

 dy = (6x + 15) dx

Δy = (6x + 15) Δx.

Let, x = 3 and Δx = 0.02 then,

Δy = f (x + Δx) - f (x)

 f (x + Δx) = f (x) + Δy = f (x) + (6x + 5) Δx.

f (3 + 0.02) = 3 (3)2 + 15 (3) + 5 + (6 × 3 + 15) (0.02).

 f (3.02) = 27 + 45 + 5 + (18 + 15) (0.02).

= 77 + 0.66

= 77.66

∴ Option (D) is correct.

Q:  

73. The approximate change in the volume of a cube of side x metres caused by increasing the side by 3% is (A) 0.06 x3 m3 (B) 0.6 x3 m3  (C) 0.09 x3 m3 (D) 0.9 x3 m3

Read more
A: 

The volume v of a cube with side ‘x’ metre is v = x3

So,  dv= (dvdx)Δx=3x2Δx.

∴increase in side, Δx = 3% of = 3x100.

∴dv = 3x2π 3x1000.09x3 m3.

Hence, option (C) is correct.

Q:  

75. Find the maximum and minimum values, if any, of the following functions given by

(i) f(x) = |x + 2| - 1

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A: 

(i) we have, f(x) = |x + 2| - 1

We know that, for all x?,|x+2|0

|x+2|11.

 f(x)- 1.

∴ Minimum value of f(x) = -1 when x + 2 = 0 x = - 2.

And maximum value of f(x) does not exist.

(ii) g(x)=|x+1|+3

A(ii)

We have, g(x)=|x+1|+3

For all x,|x+1|0.

|x+1|0

|x+1|+30+3

 g(x) 3.

∴ Maximum value of g(x) = 3 when |x+1|=0x=1.

And minimum value does not exist.

(iii) h(x) = sin (2x) + 5.

A(iii)

we have, h(x) = sin (2x) + 5.

For all x?,1sin2x1. {range of sine function is [-1, 1]}

-1 + 5 sin 2x + 5 1 + 5.

 4 h(x) 6.

∴ Maximum value of h(x) = 6.

Minimum value of h(x) = 4.

(iv) f(x)=|sin4x+3|.

A(iv)

we have, f(x)=|sin4x+3|.

As for all x?,1sin4x1

-1 + 3 sin 4x + 3 1 + 3

|2||sin4x+3||4|.

 2 f(x) 4.

∴ Maximum value of f(x) = 4.

Minimum value of f(x) = 2.

(v) h(x) = x + 1.,x∈ ( -1, 1).

A(v)

we have, h(x) = x + 1.,x∈ ( -1, 1).

Given, -1

- 1 + 1 <x + 1 < 1 + 1

0 <h (x) < 2.

∴ Maximum value and minimum value of h(x) does not exist.

Q:  

76. Find the local maxima and local minima, if any, of the following functions.

       Find also the local maximum and the local minimum values, as the case may be:

(i) f(x) = x2.

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A: 

(i) we have, f(x) = x2.

f(x) = 2x.

andf(x) = 2.

AR extreme point, f(x) = 0

2x = 0

x = 0.

When x = 0, f(0) = 2 > 0.

∴x = 0 is a point of local minima and value of local minimum is given by f(0) = 02 = 0.

(ii) g(x) = x3 3x

A(ii)

we have, g(x) = x3- 3x

g'(x) = 3x2- 3

g''(x) = 6x.

At extreme point,

g'(x) = 0

3x2- 3 = 0.

3(x2- 1) = 0 ⇒ 3(x - 1)(x + 1) = 0.

x = 1 or x = -1.

At x = 1, g"(1) = 6.1 = 6 > 0.

So, x = 1 is a point of local minima and value of local minimum is given by g(1) = 13- 3.1 = 1 - 3 = - 2.

And at x = -1, g"( -1) = 6 ( -1) = 6 < 0.

So, x = -1 is a point of local minima and value of local minimum is given by

g(- 1) = (- 1)3- 3(- 1) = 1 + 3 = 2.

(iii) h(x)=sinx+cosx.0<x<π2

A(iii)

we have, h(x) = sin x + cos x.

h'(x) = cos x - sin x

h'(x) = -sin x - cos x.

At extreme points,

h'(x) = 0

cosx - sin x = 0

cosx = sin x

cosxcosx=sinxcosx

 1 = tan x

tanx=tanπ4

x=π4.

(iv) f(x)=sinxcosx,0<x<2π

A(iv)

we have, f(x) = sin x cos x.

 f(x) = cos x + sin x

 f(x) = sin x + cos x.

At extreme points,

f(x) = 0

 cosx + sin x = 0.

sinxcosx=cosxcosx

tanx=1=tanπ4=tan(ππ4)=tan(2ππ4)

x=(ππ4) IInd quadrate or (2ππ4) IIIth quadrate

x=4ππ48xπ4

x=3π4orx=fπ4.

Atx=3π4,f(3π4)=sin3π4+cos3π4

=sin(ππ4)+cos(ππ4)

=sinπ4cotπ4

(v) f(x) = x3- 6x2 + 9x + 15.

A(v)

we have, f(x) = x3- 6x2 + 9x + 15.

f(x) = 3x2- 12x + 9.

f'(x) = 6x - 12.

At extreme point, f'(x) = 0.

3x2- 12x + 9 = 0.

x2- 4x + 3 = 0

x2-x - 3x + 3 = 0

x(x - 1) -3(x - 1) = 0

(x - 1)(x - 3) = 0

 x = 1 or x = 3.

At, x = 1,f"(1) = 6 × 1 - 12 = 6 - 12 = - 6 < 0

∴x = 1 is a point of local maxima and the value of local

Maximum is given by f(1) = 13- 6(1)2 + 9(1) + 15

= 1 - 6 + 9 + 15

= 19.

And at x = 3,f"(3) = 6 × 3 - 12 = 18 - 12 = 6 > 0

∴x = 3 is a point of local minima and the value of

local minimum is given by f (3) = 33- 6(3)2 + 9(3) + 15.

= 27 - 54 + 27 + 15.

= 15.

(vi) g(x)=x2+2x,x>0

A(vi)

We have, g(x)=x2+2x,x>0

g(x)=122x2

ggz(x)=x242x2=(x2)(x+2)2x2

g(x)=4x3.

At, extreme point, g'(x) = 0

(x+2)(x2)x2=0

x = -2 or x = 2

Given, that x > 0, hence we have x = 2.

At, x = 2, g?(x)=423=48=12>0

∴x = 2 is a point of local minima and value of local

Minimum is given by g(2) = 22+22=1+1=2.

(vii) f(x)=1x2+2

A(vii)

Given, f(x)=1x2+2

g(x)=1(x2+2)2ddx(x2+2)=2x(x2+2)2.

g(x)=[(x2+2)2ddx2x2xddx(x2+2)2(x2+2)4]

=[2(x2+2)22x2(x2+2)2x](x2+2)4.

=[2(x2+2)2[x2+24x2]](x2+2)4

=2(x2+2)(23x2)(x2+2)4=2(23x2)(x2+2)3.

At extreme points, g'(x) = 0.

2x(x2+2)2=0.

 x = 0.

At, x = 0, g(x)=2(22×02)(02+2)3=2(20)23=12<0

∴x = 0 is a point of local maxima and value of local maximum is given by g(0)=102+2=12.

Q:  

78. Find the absolute maximum value and the absolute minimum value of the following functions in the given intervals:

(i) f(x) =x3 , x ∈ [– 2, 2]

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A: 

(i) We have,

f(x) = x3 , x ∈ [– 2, 2].

f(x) = 3x2.

At, f(x) = 0

3x2 = 0

x = 0 <--[-2, 2].

We shall absolute the value of f at x = 0 and points of interval [ -2, 2]. So,

f(0) = 0

f(- 2) = (- 2)3 = 8

f(2) = 23 = 8.

∴ Absolute maximum value of f(x) = 8 at x = 2

and absolute minimum value of f(x) = -8 at x = -2.

(ii) f (x) = sin x + cos x , x ∈ [0, π]

A.(ii)

We have, f(x) = sin x + cos x , x ∈ [0, π]

f(x) = cos x - sin x.

atf(x) = 0

cosx - sin x = 0

 sinx = cos x

sin°xcorx=1tanx=1tanx=tanπ4

x=π4[0,π]

(iii) f(x) = 4x 12x2,x[2,92]

A.(iii)

We have, f(x) = 4x 12x2,x[2,92]

f(x) = 4 - x

atf(x) = 0

4-  x = 0

 x = 4 [−2,92]

f(4)=4(4)12(4)2=168=8.

f(2)=4(2)12(2)2=82=10.

f(92)=4(92)12(92)2=18818=94818=638.

= 7.87.5

Hence, absolute maximum value of f(x) = 8 at x = 4

and absolute minimum value of f(x) = -10 at x = -2.

(iv) f(x) = (x - 1)2 + 3, x∈[ -3, 1]

A.(iv)

We have, f(x) = (x - 1)2 + 3, x∈[ -3, 1]

f(x) = 2(x -1)

atf(x) = 0

2(x - 1) = 0

x = 1 [ -3, 1]

∴f(1) = (1 - 1)2 + 3 = 3

f(- 3) = (- 3 - 1)2 + 3 = 16 + 3 = 1

∴ Absolute maximum value of f(x) = 19 at x = -3.

And absolute minimum value of f(x) = 3 at x = 1.

Q:  

79. Find the ma proximumfit that a company can make, if the profit function is given by p(x) = 41 – 72x – 18x2

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A: 

We have, p (x) = 41 -f2x - 18x2.

P (x) = - 72 - 36x

P (x) = -36

At extreme point,

- 72 - 36x = 0

 x=7236=2 .

At x = - 2, p" (x) = - 36 < 0.

∴x = -2 is a point of local maximum and the value of local

Maximum is given by P (2) = 41 - 72 (- 2) - 18 (- 2)2

41 + 144 - 72 = 113 units.

Q:  

80. Find both the maximum value and the minimum value of 3x4 – 8x3 + 12x2 – 48x + 25  on the interval [0, 3].

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A: 

We have, f (x) = 3x4- 8x3 + 12x2- 48x + 25, x ∈  [0, 3].

f (x) = 12x3- 24x2 + 24x - 48.

At f (x) = 0.

12x3- 24x2 + 24x - 48 = 0.

x3- 2x2 + 2x - 4 = 0

x2 (x - 2) + 2 (x - 2) = 0

(x - 2) + (x2 + 2) = 0

x = 2 ∈ [0, 3] or x = ±√-2 which is not possible as

∴f (x) = 3 (2)4- 8 (2)3 + 12 (2)2- 4 (2) + 25.

=48 - 64 + 48 - 96 + 25.

= -39.

f (0) =3 (0)4- 8 (0)3 + 12 (0)2- 48 (0) + 25.

= 25.

f (3) = 3 (3)4- 8 (3)3 + 12 (3)2- 48 (3) + 25.

= 243 - 216 + 108 - 144 + 25

= 16.

Maximum value of f (x) = 25 at x = 0.

and minimum value of f (x) = -39 at x = 2.

Q:  

81. At what points in the interval [0, 2π], does the function sin 2x attain its maximum value?

A: 

We have, f(x) = sin 2x, x ∈ [0, 2π],

f(x) = 2cos 2x.

At f(x) = 0.

2 cos 2x = 0

cos 2x = 0

2x=(2x+1)π2,x=0,1,2,3.

x=(2x+1)π4.

x=π4,3π4,5π4,7π4,[0,2π]

f(π4)=sin2π4=sinπ2=1 .

f(3π4)=sin2(3π4)= sin3π2 f(7π4)

=sin(π+π2)= =sin2×7π4

=sinπ2 =sin7π2

= 1. =sin3π+π2

f(5π4)=sin2×(5π4)=sin5π2=sin(2x+π4) =sinπ2

=sinπ4=1 = 1.

f(0) = sin 2(0) = sin 0 = 0

f(2π) = sin 2(2π) = sin 4π = 0

Hence, the points of maximum xfx are.

(π4,2)and (5π4,1).

Q:  
83. Find the maximum value of 2x3 – 24x + 107 in the interval [1, 3].

Find the maximum value of the same function in [–3, –1]

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A: 

We have, f (x) =2x3- 24x + 107, x [1,3]

f (x) = 6x2- 24.

At f (x) = 0

6x2- 24= 0

x2=246=4

x = ±2. ->x = 2 ∈ [1, 3].

So, f (2) = 2 (2)3- 24 (2) + 107 = 16 - 48 + 107 = 75.

f (1) = 2 (1)3- 24 (1) + 107 = 2 - 24 + 107 = 85.

f (3) = 2 (3)3- 24 (3) + 107 = 54 - 72 + 107 = 89

∴ Maximum value of f (x) in interval [1, 3] is 89 at x = 3.

When x ∈  [ -3, -1]

From f (x) = 0

x = -2 ∈ [ -3, -1]

So, f (- 2) = 2 (- 2)3- 24 (- 2) + 107 = - 16 + 48 + 107 = 139.

f (- 3) = 2 (- 3)3- 24 (- 3) + 107 = - 54 + 72 + 107 = 125.

f (- 1) = 2 (- 1)3- 24 (- 1) + 107 = - 2 + 24 + 107 = 129.

∴ Maximum value of f (x) in interval [ -3, -1] is 139 at x = -2.

Q:  

84. It is given that at x = 1, the function x4 – 62x2 + ax + 9 attains its maximum value, on the interval [0, 2]. Find the value of a.

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A: 

We have, f (x) = x4- 62x2 + ax + 9, x∈  [0, 2].

f (x) = 4x3- 124x + a

∴f (x) active its maxn value at x = 1 [0, 2]

∴f (1) = 0.

4 (1)3- 124 (1) + a = 0

a = 124 - 4 = 120.

∴a = 120

Q:  

86. Find two numbers whose sum is 24 and whose product is as large as possible.

A: 

Let ‘x’ and ‘y’ be the two number

Then, x + y = 24  y = 24 - x

Let ‘P’ be their product then,

P = xy = x (24 - x) = 24x -x2

P (x) = 24x -x2

dpdx=242x.

d2pdx2=2.

At dpdx=0

24 - 2x = 0

x=242=12.

So, P (12) d2pdx2=2<0.

x = 12 is a point of local maxima

Hence, y = 24 - 12 = 12.

The uqdtwno (x, y) is (12, 12).

Q:  

87. Find two positive numbers x and y such that x + y = 60 and xy3 is maximum.

A: 

We have, x + y = 60.where x, y > 0

x = 60 - y.

Let the product P = xy3 = (60 -y) y3 = 60y3-y4

dpdy=180y24y3

= 4y2 (45 -y)

At dpdy=0

4y2 (45 -y) = 0

 y = 0 and y = 45

As y > 0, y = 45

When, y > 45,  dpdy= (+ve) (ve)

= ve < 0

Ad y < 45,  dpdy= (+ve) (+ve)

= (+ ve) > 0

∴p is maximum when y = 45 from + ve to- ve or y increases through 45.

So, x = 60 - y = 6Ø - 45 = 15.

Øx = 15 and y = 45.

Q:  

88. Find two positive numbers x and y such that their sum is 35 and the product x2 y5 is a maximum.

A: 

We have, x + y = 35.

y = 35 - x

Let the product, P =x2 y5

P = x2 (35 -x)5

So,  dpdx = x2 5 (35 -x)4 (1) + (35 -x)5 2x

= x (35 -x)4 [ - 5x + (35 -x) 2]

= x (35 -x)4 [ - 5x + 70 - 2x]

= x (35 -x)4 (70 - 7x)

= 7x (35 -x)4 (10 -x)

At dpdx=0

7x (35 -x)4 (10 -x) = 0

 x = 0, 35, 10

As x is a (+) ve number we have only

x = 10, 35

And again (at x = 35) y = 35 = 0 but yis also a (+) ve number

we get, x = 10 (only)

whenx < 10,

? dpdx= (+ve) (+ve) (+ve)>0

and when x > 10,

dpdx= (+ve)ve (+ve) (ve)= (ve)<0

dpdx changes from (+ ve) to ( -ve) as x increases while passing through 10

Hence, x = 10 is a point of local maxima

So, y = 35 - 10 = 25

∴x = 10 and y = 25

Q:  

89. Find two positive numbers whose sum is 16 and the sum of whose cubes is minimum

A: 

Let, x and y be the two positive number

Then, x + y = 16 y = 16 - x

Let p be the sun of the cubes then

p = x3 + y3 = x3 + (16 -x)3 = x3 + (16)3-x3- 48x (16 -x)

p = 163 + 48x2- 76 8x

So,  dpdx=96x768.

d2pdx2=96.

At dpdx=0

96x - 768 = 0

x=76896=8.

∴at x = 8,  d2pdx2=96>0

So, x = 8 is a point of local minima

So, y = 16 - 8 = 8

Hence, x = 8, y = 8

Q:  

90. A square piece of tin of side 18 cm is to be made into a box without top, by cutting a square from each corner and folding up the flaps to form the box. What should be the side of the square to be cut off so that the volume of the box is the maximum possible.

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A: 

Side of the tin square piece = 18 cm

Let x cm be the thought of the square to be cut from each corner.

The volume v of the box after cutting

v = length × breadth × height

= (18 - 2x) (18 - 2xx x

= (18 - 2x)2x

= (324 + 4x2- 72x) x

= 4x3- 72x2 + 324x

So,  ddx=12x2144x+324

d2vdx2=24x144

As ddx=012x2144x+324=0

x2- 12x + 27 = 0

x2- 9x- 3x + 27 = 0

x (x - 9) - 3 (x - 9) = 0

(x - 9) (x - 3) = 0

 x = 9 and x = 3

At x = 0, length of box = 18 - 2π9 = 18 - 18 = 0

Which is not possible

And at x = 3,  d2ydx2 = 24 (3) - 144 = -72 < 0

∴x = 3 is a point of maximum

Hence, ‘3’ cm (square) is to be cut from each side of the square

So that volume of box is maximum

Q:  

91. A rectangular sheet of tin 45 cm by 24 cm is to be made into a box without top, by cutting off square from each corner and folding up the flaps. What should be the side of the square to be cut off so that the volume of the box is maximum?

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A: 

Let ‘x’ cm be the length of side of the square to be cut off from the rectangular surface

Then, the volume v of the box is v = (45 - 2x) (24 - 2x) x

= 1080x - 138x2 + 4x3

So,  dvdx=1080276x+12x2

d2dx2=276+24x

At,  dvdx=01080276x+12x2=0

x2- 23x + 90 = 0

x2- 5x - 18x + 90 = 0

x (x - 5) - 18 (x - 5) = 0

(x- 5) (x- 18) = 0

 x = 5 and x = 18

At x = 18, breadth = 24 - 2 (18) = 24 - 36 = -12 which is not possible

At,  x=5, d2vdx2=276+24 (5)=276+120=150<0

Hence, x = 5 is the point of maximum

So, ‘5’ cm length of square seeds to be cut from each corner of the secthgle

Q:  

93. Show that the right circular cylinder of given surface and maximum volume is such that its height is equal to the diameter of the base.

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A: 

Let r and h be the radius and height of the cylinder

So, r, h > 0

The total surface area s is given by

S = 2πr(h + r) = content .

S2π=r(h+r) = content = x (say)

h+r=ur

h=krr=kr2r.

Then, the volume v of the cylinder

V=πr2h=πr2(kr2r)=πr(xx2)=π(kxπ3)

So, dvdr=π(x3r2)

d2dr2=6πr.

For maximum, dvdr=0

π[x3x2)=0

r2=x3

Q:  

94. Of all the closed cylindrical cans (right circular), of a given volume of 100 cubic centimetres, find the dimensions of the can which has the minimum surface area?

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A: 

The volume v of a cylinder of height h and radius r is

V = πr2h = 100

h=100πr2.

Let, s be the surface area then

S = 2r2hr(r + h) = 2x(r2+100rπr2)

=2π(n2+100πn).

dSdr=2π[2x1001r2]

d2Sdr2=2π[2+200πr3]

At, dsdr=2π[2r100πr2]=0

2r=100πr2

⇒M3=50πM=[50π]13,>0.

At, x=[50π]13,d2Sdr2=2π[2+200π[50π]13×3]

=2π[2+4]=12π>0

r=[50π]13 isa point of minimum

And h=100π[50π]23=2[50π]13.

Q:  

95. A wire of length 28 m is to be cut into two pieces. One of the pieces is to be made into a square and the other into a circle. What should be the length of the two pieces so that the combined area of the square and the circle is minimum?

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A: 

Let x and y in ‘m’ be the length of side of the square the radius of the circle respectily

Then, length of wire = perimeter of square + circumference of circle

28 = 4x + 2πy

 2x + πy = 14

y=142xx

The combine area A of the square and the circle is

A = x2 + πy2

=x2+π[142xπ]2

=x2+[2(7x)x]2

=x2+4π(7x)2.

So, dAdx=2x+42π(7x)+(1)=2x8π(7x)

d2Adx2=2+8π

At, dAdx=0

2x8(7x)π=0

2x=8π(7x)

2πx=568x

(2x+8)x=56

x=562π+8=28x+4.

At, x=28π+4,d2Adx2=2+γπ>0

x=28π+4 isa point of minima

Hence, length of square = 4x=4(281+4)=112π+4m

and length of circle = 2πy

=2π(142xπ)

=2π[14π21[28π+4]]

=284[28π+4]=281121+4

=28x+112112x+4

=28ππ+4

Q:  

96. Prove that the volume of the largest cone that can be inscribed in a sphere of radius R is 827 of the volume of the sphere.

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A: 

Let r and h be the radius and height of the one in scribed in the sphere of radius R.

Then, is ΔOBC, rt angle at B (h-r)2 + r2 = R2

h2 + R2- 2hR + h2 = R2

r2 = 2hR -h2

Then the volume v of the cone is, V=13×r2h =13π(2hRh2)h

=13π(2Rh2h3).

dVdh=π3(4Rh3h2).

d2Vdh2=π3(4R6h).

At dVdh=0

π3(4Rh3h2)=0.

4Rh – 3h2 = 0.

h(4R – 3h) = 0.

h = 0 and h=4R3

As h> 0, h=43.

At h=4R3,d2vdh2=π3[4R6×4rR3]

=13[4R8R]=43<0

h=4R3 is a point of maxima.

and r2=2hRh2=2×4R3R−(4R3)2

=3×8R2916R29

r2=8R29

Hence, Volume of Cone, =13πr2h

=13π×8R29×4R3

=827×43πR3=827× Volume of sphere.

Q:  

98. Show that the semi-vertical angle of the cone of the maximum volume and of given slant height is tan1 √2.

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A: 

Let r, h, l and Ø be the radius, height, slant height and semi-vertical angle respectively of the cone. i.e., r, h, l>0.

Then, Volume V of the cone is

=13πr2h. 

=13π (l2h2)h

=13π (l2hh3).

So,  dVdh=13π (l23h2)

d2Vdh2=2πh

Q:  

100. The point on the curve x2 = 2y which is nearest to the point (0, 5) is

(A) (2√2  ,4) (B) (2√2  ,0) (C) (0, 0) (D) (2, 2)

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A: 

The equation of the given curve is

x2 = 2y.

Let p (x, y) be a point on the curve.

The distance of p (x, y) from (0, 5) is say S is given by

 

s2=x2+ (y5)2

Let z = s2 = x2 + y2 + 25 – 10y = 2y + y2 -10y + 25

z = y2 – 8y + 25

So,  dzdy=2y8

d2zdy2=2

At dzdy=02y8=0y=82=4

At y = 4,  d2zdy2=2>0

y = 4, is point of minimum distance.

So, x2 = 2y->x2 = 2 × 4-> x2 = 8 x=±2

Hence, the point of the nearest distance are  (2√2, 4) and

(2√2, 4).

Option (A) is correct.

Q:  

101. For all real values of x, the minimum value of 1x+x21+x+x2 is

(A) 0 (B) 1 (C) 3 (D) 13

A: 

We have,

f(x)=1x+x21+x+x2.

f(x)=(1+x+x2)ddx(1x+x2)(1x+x2)ddx(1+x+2)(1+x+x2)2

f(x)=(1+x+x2)(1+2x)(1x+x2)(1+2x)(1+x+x2)2

=1+2xx+2x2x2+2x312x+x+2x2x22x3(1+x+x2)2

=2+2x2(1+x+x2)2=2(1x2)(1+x+x2)2=2(1+x)(1x)(1+x+x2)2

At f(x) = 0.

2(1+x)(1x)(1+x+x2)2=0

x = 1 and x = -1.

At x=1,f(1)=11+11+1+1=13 x=1,f(1)=11+11+1+1=13

At x = -1, f(1)=1+1+111+1=3

The maximum value of f(x) =13x=1

Hence, option (D) is correct.

Q:  

102. The maximum value of [x(x1)+1]13 , 0

(A) 1313 (B) 12 (C) 1 (D) 0

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A: 

We have,

f(x)=[X(x - 1)+
1]13,0x1

f(x)=13[x2x+1]23ddx(x2x+1)

=2x13(x2x+1)23

At f(x) = 0.

2x – 1 = 0

x=12[0,1]

f(12)=[12(121)+1]13=[1412+1]13.

=[12+44]13

[34]13?0.90

f(0)=[0(0 - 1)
+1]13=1.

f(1)=[1(1 - 1)+1]
13=1.

Option (B) is

Hence maximum value of f(x) = at x = 1 and x = 0.

Option (c) is correct.

Q:  

103. Using differentials, find the approximate value of each of the following

(a) (1781)14

A: 

(a) Consider y=x14.Let,x=1681&?x=181.

Then,?y=(x+?x)14x14=(1781)14(1681)14=(1781)1423(1781)14=23+?y

Now, dy is approximately equal to ?y and is given by,

dy=(dydx)?x=14(x)34(?x)(as,y=x14)=14(1681)34(181)=274×8×181=132×3=196=0.010

Hence, the approximate value of (1781)14 is 23+0.010=0.667+0.010

=0.677

(b) (33)15

(b) Consider y=x15.Let,x=32&?x=1

Now, dy is approximately equal to ?y and is given by,

dy=(dydx)(?x)=15(x)65(?x)(as,y=x15)=15(x)6(1)=1320=0.003

Hence, the approximate value of 3315
is 12+(0.003=0.497)

Q:  

104. Show that the function given by f(x)logxx, has maximum at x = e.

A: 

We have, f(x) logxx,x>0.

f(x) = xddxlogxlogxddxx_x2.

=x×1xlogxx2=1logxx2

f(x) = x2ddx(1logx)(1logx)ddxx2x4

x2(1x)(1logx)(2x)x4

12+2logxx3 = 2logx3x3

At extreme points, f(x) = 0.

1logxx2=0. 

logx=1=loge.

x=e.

At x = e, f"(e) = 2loge3e3=23e3=1e3<0

x = e is a point of maximum.

Q:  

105. The two equal sides of an isosceles triangle with fixed base b are decreasing at the rate of 3 cm per second. How fast is the area decreasing when the two equal sides are equal to the base ?

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A: 

Let ‘b’ and ‘x’ be the fixed base and equal side of isosceales triangle.

Then,  dxdt=3 cm/s (Ø decreasing).

Q:  

106. Find the equation of the normal to curve x2 = 4y which passes through the point (1, 2)

A: 

Equation of the curve is y2=4x.......... (i)

2ydydx=4dydx=42y=2ydydx] (1, 2)=22=1

Now, the slope of the normal at point  (1, 2) is 1dydx] (1, 2)=11=1

 Equation of the normal at  (1, 2) is y2=1 (x1).

y2=x+1x+y3=0

Q:  

107. Show that the normal at any point θ to the curve x = a cosθ + a θ sin θ, y = a sinθ – aθ cosθ is at a constant distance from the origin.

Read more
A: 

We have x=acosθ+aθsinθ

dxdθ=asinθ+asinθ+aθcosθ=aθcosθy=asinθaθcosθdydθ=acosθacosθ+aθsinθ=aθsinθdydx=dydθ.dθdx=aθsinθaθcosθ=tanθ

 Slope of the normal at any point θ is 1tanθ

The equation of the normal at a given point (x,y) is given by,

yasinθ+aθcosθ=1tanθ(xacosθaθsinθ)ysinθasin2θ+aθsinθcosθ=xcosθ+acos2θ+aθsinθcosθxcosθ+ysinθa(sin2θ+cos2θ)=0xcosθ+ysinθa=0

Now, the perpendicular distance of the normal from the origin is

Q:  
108. Find the intervals in which the function f given by

f(x)= 4sinx2xxcosx.2+cosx is (i) increasing (ii) decreasing

Read more
A: 

We have, f(x)= 4sinx2xxcosx.2+cosx

=4sinxx(2+cosx)2+cosx

f(x)=4sinx2+cosxx.

So, f(x)=(2+cosx)ddx(4sinx)4sinxddx(2+cosx)(2+cosx)2dxdx.

=(2+cosx)(4cosx)(4sinx)(sinx)(2+cosx)21.

=8cosx+4cos2x+4sin2x(2+cosx)21

=8cosx+4(cos2x+sin2x)(2+cosx)21.

=8cosx+4(2+cosx)21.

=8cosx+4(2+cosx)2(2+cosx)2

=8cosx+444cosxcos2x(2+cosx)2

=4cosxcos2x(2+cosx)2=cosx(4cosx)(2+cosx)2.

Now, (2+cosx)2>0.

And, 4cosx>0 as cos x lies in [1, 1].

So, (i) for increasing, f(x) ≥ 0.

cosx ≥ 0.

x lies in Ist and IVth quadrant.

i.e., f(x) is increasing for 0xx2 and 3x2x2x. 

(ii) for decreasing, f(x) ≤ 0.

cosx ≤ 0.

x lies in IInd and IIIrd quadrant.

i.e., f(x) is decreasing for x2x3x2 .

Q:  

109. Find the intervals in which the function f given by f(x)=x3+1x3,x0 is

(i) Increasing (ii) Decreasing

A: 

We have, f(x) = x3+1x3,x0.

f(x)=3x23×1x4=3(x21x4)=3(x61x4).

=3x4[(x2)313]

=3x4(x21)(x4+x2+1) { (a3b3)= (ab)(a2+b2+ab)

f(x)=3(x1)x4(x+1)(x4+x2+1). {?x2b2=(ab) (a+b)

At f(x)=0.

3(x1)(x+1)(x4+x2+1)x4=0

x=1x=1. 3(x4+x2+1)0. 

So we have three disjoint internal i.e.,

(,1],[1,1](1,). 

When, x(,1].

f(x)=(+)ve(ve)(ve)(+ve)(+ve)=(+)ve  and0  at x=1. 

So, f(x) is increasing.

When  x[1,1]

f(x)=(+ve)(v)(+ve)=(ve) 0 atx=1and 1

So, f(x) is decreasing.

When x[1,)

f(x) =  (+ve)(+ve) (+ve)=( +ve) on 0 at x=1

So, f(x) is increasing.

f(x) is increasing for x(∞,1) and [1, ∞] and decreasing for x[1, 1].

Q:  

110. Find the maximum area of an isosceles triangle inscribed in the ellipse

x2a2+y2b2=1 with its vertex at one end of the major axis.

Read more
A: 

The given equation of the ellipse is x2a2+y2b2=1  (1)

Let the major axis be along x-axis so, vertex is at  (±a, 0)

Let ΔABC be the isosceles triangle inscribed on the

ellipse with one vertex C at (a, 0).

Then, let A have Co-ordinate (x0, yo) from figure.

So, Co-ordinate of B = (x0, y0)

As A and B lies on the ellipse, from equation (i),

Q:  

111. A tank with rectangular base and rectangular sides, open at the top is to be constructed so that its depth is 2 m and volume is 8 m3. If building of tank costs Rs 70 per sq metres for the base and Rs 45 per square metre for sides. What is the cost of least expensive tank?

Read more
A: 

Let x and y meters be the length and breath of the rectangular base of the tank respectively.

Then, volume V of the tank is

V = length × depth × breath.

V = 2xy = 8m3(given).

y=82x=4x.

Let ‘t’ be the total cost of building the tank.

Then, t = cost of base + cost of sides.

= 70xy + 45[4x+4y]     {there are four sides.

= 70xy + 180x+ 180y.

=70x4x+180x+180×4x.

t=280+180x+720x.

So, dzdx=0+180720x2.

And d2tdx2=1440x3

At d2dx=0180720x2=0

x2=720180=4

x = ± 2

x = 2, (x = length and it cannot be negative)

At x = 2, d2tdx2=144023=14408=180>0.

x = 2 is point of maxima.

Hence, minimum cost = 280+180×2+7202 = 280 + 360 + 360 = 1000.

Q:  

112. The sum of the perimeter of a circle and square is k, where k is some constant. Prove that the sum of their areas is least when the side of square is double the radius of the circle.

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A: 

Let r and s be the radius of the circle and length of side of square.

Then, sum of perimeter of circle and square = k

2πr +4s = k

s = k2×r4

The area A be the total areas of the circle and square.

Then, A = πr2 + s2

=πr2+(k2πr4)2=πr2+x2+4π2r24πrk16

=16πr2+x2+4π2r24πkr16

=116[(16π+4π2)π24πkr+x2].

So, dAdr=116[(16π+4x2)2n4πk.].

And d2Adr2=116[(6π+4π2)2]=16π+4π28

At dAdr=0

116[(16x+4x2)2x4πk]=0

(16x+4x2)2x4πk=0

4π[4+π]2r=4πk.

r=k2(4+π).

At r=x2(4+π),d2Adx2=16π+4π28>0.

s = 2x2(4+x).

s = 2r.

Hence, proved.

Q:  

113. A window is in the form of a rectangle surmounted by a semicircular opening. The total perimeter of the window is 10 m. Find the dimensions of the window to admit maximum light through the whole opening.

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A: 

Let ‘x’ metre be the radius of the semi-circular opening mounded on the length ‘2x’ side of rectangle. Then, let ‘y’ be the breadth of the rectangle.

Then, perimeter of the window = 10m

2πx2+2x+y+y=10

x + 2x + 2y + = 10.

y=10(π+2)π2.

Let the area of the window be A.

Then, A = 12(πx2)+2xy.

=πx22+2x·[10(x+2)x2].

=πx2+20x2πx24x22

12 [-πx2- 4x2 + 20x].

So, ddx=12 [ -2πx - 8x + 20]

And d2dx2=12 [ -2π - 8] = -π -4 = -( π+ 4)

At ddx=0.

12 [ -2πx - 8x + 20] = 0

2x + 8x = 20

x = 202π+8 = 10π+4.

At x = 10π+4,d2Adx2 = -( π+ 4) < 0

Øx = 10x+4 is a point of minima.

And y = 10(x+2)×(10x+4)2

=10(π+4)(π+2)102(π+4)

=10π+4010π202(x+4)=10π+4.

Ø Dimensions of the window are

length = 2x = 20π+4m

breadth = y 10π+4m.

radius = y = 10π+4m.

Q:  

114. A point on the hypotenuse of a triangle is at distance a and b from the sides of the triangle. Show that the minimum length of the hypotenuse is

(a23+b23)32 .

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A: 

Let P be the point on hypotenuse of a triangle. ABC, t angle at B.

Which is at distance a& b from the sides of the triangle.

Let < BAC = < MPC = Ø.

Then, in… ΔANP,

tan3θ=ab

tanθ=(ab)13.

At tan Ø = (ab)13,

d2q?dθ2=+ve.>0. { Øall trigonometric fxn are + ve in Ist quadrant}.

So, z is least for tanØ = (ab)13.

As, Sec2Ø = 1 + tan2Ø = 1 + (ab)23. = b23+a23b23.

secØ = [b23+a23b23]12 = (a23+b23)12b13.

And tan2Ø = 1cot2θ

cot2Ø = (ab)23

And cosec2Ø = 1 + cot2Ø = 1 + (ab)23 = a23+b23a23

cosecØ = (a23+b23)12.a13

Q:  

115. Find the points at which the function f given by f (x) = (x – 2)4 (x + 1)3 has (i) local maxima (ii) local minima (iii) point of inflexion

Read more
A: 

We have,

f (x) = (x- 2)4 (x + 1)3.

So, f (x) = (x- 2)4. 3 (x + 1)2 + (x + 1)3. 4 (x- 2)3.

= (x- 2)3 [x + 1)2 [3 (x- 2) + 4 (x + 1)]

= (x- 2)3 (x + 1)2 (3x- 6 + 4x + 4)

= (x- 2)3 (x + 1)2 (7x- 2).

At f (x) = 0.

(x- 2)3 (x + 1)2. (7x- 2) = 0.

x = 2, x = -1 or x = 27.

As (x + 1)2> 0, we shave evaluate for the remaining factor.

At x = 2,

When x< 2, f (x) = ( -ve) (+ ve) (+ ve) = ( -ve) < 0.

When x> 2, f (x) = (+ ve) (+ ve) (+ ve) = (+ ve) > 0.

Øf (x) change from ( -ve) to (+ ve) as x increases

So, x = 2 is a point of local minima

At x = -1.

When x< -1, f (x) = ( -ve) (+ ve) ( -ve) =∉, ve > 0.

When x> -1, f (x) = ( -ve) (+ ve) (+ ve) =∉, ve > 0.

So, f (x) does not change through x -1.

Hence, x = -1 is a point of infixion

At x = 27,

When x< 27, f (x) = ( -ve) (+ ve) ( -ve) = ( -ve > 0.

When x< 27, f (x) = ( -ve) (+ ve) (+ ve) = ( -ve < 0.

f (x) change from (+ ve) to ( -ve) as x increases through 27.

x = 27 is a point of local maxima

Q:  

116. Find the absolute maximum and minimum values of the function f given by f (x) =cos2 x + sin x, x ∈ [0, π]

Read more
A: 

We have,

f (x) = cos2x + sin x, x∈ [0, π ].

So, f (x) = 2 cos x ( -sin x) + cos x = cos x (1 - 2 sin x).

At f (x) = 0

cosx (1 - 2 sin x) = 0

cosx = 0  or  1 - 2 sin x = 0

cosx = cos π2 or sin x = 12 = sin π6 = sin xπ6

x= π2 , x = π6 and x = 5π6  [0, π ].

So, f  (π2) = cos2π2 + sin π2 = 1.

Absolute minimum of f (x) = 54 and absolute minimum of f (x) = 1.

Q:  

117. Show that the altitude of the right circular cone of maximum volume that can be inscribed in a sphere of radius r is 4r3 .

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A: 

Let x be the radius of the sphere ad x be radius of the right circular cone.

Let height of cone = y

Then, in ΔOBA,

(y-r)2 + x2 = r2

y2 + r2- 2ry + x2 = r2

x2=2ry - y2

So, the volume V of the cone is

V=13π·x2·y {=13π(radius)2×hight}

=13×(2xyy2)y

=13x(2xy2y3).

So, dvdy=π3[4xy3y2]

And d2dy2=x3[4x6y].

At dVdy=0.

π3[4xy3y2]=0

4x -y- 3y2 = 0

y=4x3 asy> 0.

At y = 4π3,d2vdy2=π3[4x8x] = 4πr3<0.

Ø V is maximum when y = 4π3

Q:  

118. Let f be a function defined on [a, b] such that f ′(x) > 0, for all x ∈ (a, b).

Then prove that f is an increasing function on (a, b).

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A: 

We have,

f (x) defined on [a, b]

And f (x) > 0 ∀ x ∈ [a, b].

Let x1, x2 ∈ [a, b] and x2>x1

In the internal x1, x2], f (x) will also be continuous and differentiable.

Hence by mean value theorem, there exist c [x1, x2] such that

f (c)=f (x2)f (x1)x2x1.

f (x) > 0 ∀ x ∈ [a, b].

Then, f (c) > 0.

f (x2)f (x1)x2x1>0

i.e., f (x2) -f (x1) > 0

f (x2) >f (x1).

Hence, the function f (x) is always increasing on [a, b]

Q:  

119. Show that the height of the cylinder of maximum volume that can be inscribed in a sphere of radius R is 2R/√3. Also find the maximum volume.

Read more
A: 

Kindly go through the solution

Q:  

121. A cylindrical tank of radius 10 m is being filled with wheat at the rate of 314 cubic metre per hour. Then the depth of the wheat is increasing at the rate of (A) 1 m/h (B) 0.1 m/h (C) 1.1 m/h (D) 0.5 m/h

Read more
A: 

Let x be the depth of the wheat inside the cylindrical tank is with radius = 10 cm

Then, Volume V of the cylindrical tank is

V = π (10)2x = 100πpx m3

As ddt=3143h

100πdxdt=314

dxdt=314100π=314100π3.14=314314=1.mh i e, rate of increasing of depth

option (A) is correct

Q:  

124. The normal to the curve x2 = 4y passing (1,2) is (A) x + y = 3 (B) x – y = 3 (C) x + y = 1 (D) x – y = 1

Read more
A: 

The equation of the given curve is x2=4y

Differentiating with respect to x, we have:

2x=4.dydxdydx=x2

The slope of the normal to the given curve at point (h,k) is given by,

1dydx](h,k)=2h

 Equation of the normal at point (h,k) is given as:

yk=2h(xh)

Now, it is given that the normal passes through the point (1,2).

Therefore, we have:

2k=2h(1h)or,k=2+2h(1h)..........(i)

Since (h,k) lies on the curve x2=4y ,we have h2=4k

k=h24

From equation (i), we have:

h24=2+2h(1h)h34=2h+22h=2

h3=8h=2k=h24k=1

Hence, the equation of the normal is given as:

y1=22(x2)y1=(x2)x+y=3

Therefore, option (A) is correct.

Q:  

125. The points on the curve 9y2 = x3 , where the normal to the curve makes equal intercepts with the axes are

(A) (4,±83) (B) 4,83 (C) 4,±38 (D) ±4,38

Read more
A: 

The equation of the given curve is 9y2=x3

Differentiate with respect to x, we have:

9(2y)dydx=3x2dydx=x26y

The slope of the normal to the given curve at point (x1,y1) is

1dydx](x1,y1)=6y1x12

 The equation of the normal to the curve at (x1,y1) is

yy1=6y1x12(xx1)x12yx12y1=6xy1+6x1y16xy1+x12y=6x1y1+x12y16xy16x1y1+x12y1+x12y6x1y1+x12y1=1xx1(6+x1)6+yy1(6+x1)x1

It is given that the normal makes intercepts with the axes.

Therefore, we have:

x1(6+x1)6=y1(6+x1)x1x16=y1x1x12=6y1..........(i)

Also, the point (x1,y1) lies on the curve, so we have

9y12=x13..........(ii)

From (i) and (ii), we have:

9(x126)=x13x144=x13x1=4

From (ii), we have:

9y12=(4)3=64y12=649y1=±83

Hence, the required points are (4,±83) .

Therefore, option (A) is correct.

Q:  

36. Prove that the function given by f (x) = x3 – 3x2 + 3x – 100 is increasing in R.

A: 

We have, f (x) = x3- 3x2 3x- 100.

So, f (x) = 3x2- 6x + 3 = 3 (x2- 2x + 1) = 3 (x- 1)2

For xR.

(x- 10)20 , 0 for x = 1.

3 (x- 1)2 0

 f (x) 0

∴f (x) is increasing on ?

Q:  

77. Prove that the following functions do not have maxima or minima:

(i) f(x) = ex

A: 

(i) We have, f (x) = ex

f (x) = ex.

f (x)=ex.

At, extreme points,

f (x) = 0

ex = 0 which has no real ‘a’ value

∴f (x) has with maximum or minima

(ii) g (x) = log x

A (ii)

We have, g (x) = log x,

 g (x) = 1x

At extreme points,

g (x) = 0

1x=0.

 1 = 0 which is not true.

∴g (x) was value minima or maxima

(iii) h (x) = x3 + x2 + x + 1.

A (iii)

We have, h (x) = x3 + x2 + x + 1.

h (x) = 3x2 + 2x + 1

At extreme points,

h (x) = 0

3x2 + 2x + 1 = 0

Maths Ncert Solutions class 12th Logo

NCERT Solutions for Class 12 Maths Ch 6 Application of Derivatives- FAQs

Check few of the most asked questions related to this chapter by students below;

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Class 12 Application of Derivatives 6.5 Solutions- (Old NCERT)

Class 12 Math Application of Derivatives Exercise 6.5 mainly focuses on problems such as finding minimum and maximum values of various types of functions, finding minima and maxima on a given point of functions, real real-world problems ( finding minimum and maximum profit), and finding values of volume, area, and more in a given interval for function. The Class 12 Exercise 6.5 Solutions will guide students about approaches, critical thinking skills, first and second derivative tests, and their applications in optimization. Application of Derivatives Exercise 6.5 consists of 29 Questions including 15 Long, 11 Short, and 3 MCQs. Students can check the complete solutions of the exercise 6.5 below;

Application of Derivative Exercise 6.5 Solutions

Q74.Find the maximum and minimum values, if any, of the following functions

given by.

(i)  f ( x ) = ( 2 x 1 ) 2 + 3

A(i) We have, f(x) = (2x - 1)2 + 3.

For all  x , ( 2 x 1 ) 2 0

(2x - 1)2 + 3 ≥ 3.

f(x) ≥ 3.

∴The minimum value of f(x) = 3. When 2x - 1 = 0--> x = 1 2

Again as  x , f ( x )  as there is vouppa bound to ‘x’ value hence, f(x) has no maximum values.

(ii)  f ( x ) = 9 x 2 + 1 2 x + 2

A(ii)

We have, f(x) = 92 + 12x + 2.

f ( x ) = 9 [ x 2 + 1 2 x 9 + 2 9 ]  (Taking 9 common from each team).

f ( x ) = 9 [ x 2 + 4 x 3 + 2 9 ]

f ( x ) = 9 [ x 2 + 2 × 2 x 3 + ( 2 3 ) 2 ( 2 3 ) 2 + 2 9 ]

f ( x ) = 9 [ ( x + 2 3 ) 2 4 9 + 2 9 ]

f ( x ) = 9 [ ( x + 2 3 ) 2 2 9 ] = 9 ( x + 2 3 ) 2 2

For all  x , ( x + 2 3 ) 2 0

( x + 2 3 ) 2 2 0 2

f(x)≥ - 2.

∴The minimum value of f(x) = -2 when x + 2 3 = 0

And as  x , f ( x )  so f(x) has  x = 2 3 .

no maximum values.

(iii) f(x) = (x - 1)2 + 10

A(iii)

we have, f(x) = - (x - 1)2 + 10

For all  x , ( x 1 ) 2 0 .

(x - 1)2 ≤ 0

-(x- 1)2 + 10 ≤ 10.

f(x) ≤ 10.

∴maximum value of f(x) = 10 when x - 1 = 0  x = 1.

And minimum value of f(x) does not exist.

(iv) g(x) = x3 + 1

A(iv)

we have, g(x) = x3 + 1

For the given fxn, x ,

g ( x ) a s x

and  g ( x ) a s x

Q Maximum and minimum value does not exist.

Q75. Find the maximum and minimum values, if any, of the following functions given by

(i) f(x) = |x + 2| - 1

A(i) we have, f(x) = |x + 2| - 1

We know that, for all  x , | x + 2 | 0

| x + 2 | 1 1 .

 f(x) - 1.

∴ Minimum value of f(x) = -1 when x + 2 = 0 x = - 2.

And maximum value of f(x) does not exist.

(ii)  g ( x ) = | x + 1 | + 3

A(ii)

We have,  g ( x ) = | x + 1 | + 3

For all  x , | x + 1 | 0 .

| x + 1 | 0

| x + 1 | + 3 0 + 3

 g(x) 3.

∴ Maximum value of g(x) = 3 when  | x + 1 | = 0 x = 1 .

And minimum value does not exist.

(iii) h(x) = sin (2x) + 5.

A(iii)

we have, h(x) = sin (2x) + 5.

For all  x , 1 s i n 2 x 1 . {range of sine function is [-1, 1]}

-1 + 5 sin 2x + 5 1 + 5.

 4 h(x) 6.

∴ Maximum value of h(x) = 6.

Minimum value of h(x) = 4.

(iv)  f ( x ) = | s i n 4 x + 3 | .

A(iv)

we have,  f ( x ) = | s i n 4 x + 3 | .

As for all  x , 1 s i n 4 x 1

-1 + 3 sin 4x + 3 1 + 3

| 2 | | s i n 4 x + 3 | | 4 | .

 2 f(x) 4.

∴ Maximum value of f(x) = 4.

Minimum value of f(x) = 2.

(v) h(x) = x + 1.,x∈ ( -1, 1).

A(v)

we have, h(x) = x + 1.,x∈ ( -1, 1).

Given, -1

- 1 + 1 <x + 1 < 1 + 1

0 <h (x) < 2.

∴ Maximum value and minimum value of h(x) does not exist.

Q76. Find the local maxima and local minima, if any, of the following functions.

       Find also the local maximum and the local minimum values, as the case may be:

(i) f(x) = x2.

A(i) we have, f(x) = x2.

f(x) = 2x.

andf(x) = 2.

AR extreme point, f(x) = 0

2x = 0

x = 0.

When x = 0, f(0) = 2 > 0.

∴x = 0 is a point of local minima and value of local minimum is given by f(0) = 02 = 0.

(ii) g(x) = x3 3x

A(ii)

we have, g(x) = x3- 3x

g'(x) = 3x2- 3

g''(x) = 6x.

At extreme point,

g'(x) = 0

3x2- 3 = 0.

3(x2- 1) = 0 3(x - 1)(x + 1) = 0.

x = 1 or x = -1.

At x = 1, g"(1) = 6.1 = 6 > 0.

So, x = 1 is a point of local minima and value of local minimum is given by g(1) = 13- 3.1 = 1 - 3 = - 2.

And at x = -1, g"( -1) = 6 ( -1) = 6 < 0.

So, x = -1 is a point of local minima and value of local minimum is given by

g(- 1) = (- 1)3- 3(- 1) = 1 + 3 = 2.

(iii)  h ( x ) = s i n x + c o s x . 0 < x < π 2

A(iii)

we have, h(x) = sin x + cos x.

h'(x) = cos x - sin x

h'(x) = -sin x - cos x.

At extreme points,

h'(x) = 0

cosx - sin x = 0

cosx = sin x

c o s x c o s x = s i n x c o s x

 1 = tan x

t a n x = t a n π 4

x = π 4 .

(iv)  f ( x ) = s i n x c o s x , 0 < x < 2 π

A(iv)

we have, f(x) = sin x cos x.

 f(x) = cos x + sin x

 f(x) = sin x + cos x.

At extreme points,

f(x) = 0

 cosx + sin x = 0.

s i n x c o s x = c o s x c o s x

t a n x = 1 = t a n π 4 = t a n ( π π 4 ) = t a n ( 2 π π 4 )

x = ( π π 4 )  IInd quadrate or  ( 2 π π 4 )  IIIth quadrate

x = 4 π π 4 8 x π 4

x = 3 π 4 o r x = f π 4 .

A t x = 3 π 4 , f ( 3 π 4 ) = s i n 3 π 4 + c o s 3 π 4

= s i n ( π π 4 ) + c o s ( π π 4 )

= s i n π 4 c o t π 4

(v) f(x) = x 3- 6 x 2 + 9 x + 15.

A(v)

we have, f(x) = x3- 6x2 + 9x + 15.

f(x) = 3x2- 12x + 9.

f'(x) = 6x - 12.

At extreme point, f'(x) = 0.

3x2- 12x + 9 = 0.

x2- 4x + 3 = 0

x2-x - 3x + 3 = 0

x(x - 1) -3(x - 1) = 0

(x - 1)(x - 3) = 0

 x = 1 or x = 3.

At, x = 1,f"(1) = 6 × 1 - 12 = 6 - 12 = - 6 < 0

∴x = 1 is a point of local maxima and the value of local

Maximum is given by f(1) = 13- 6(1)2 + 9(1) + 15

= 1 - 6 + 9 + 15

= 19.

And at x = 3,f"(3) = 6 × 3 - 12 = 18 - 12 = 6 > 0

∴x = 3 is a point of local minima and the value of

local minimum is given by f (3) = 33- 6(3)2 + 9(3) + 15.

= 27 - 54 + 27 + 15.

= 15.

(vi)  g ( x ) = x 2 + 2 x , x > 0

A(vi)

We have,  g ( x ) = x 2 + 2 x , x > 0

g ( x ) = 1 2 2 x 2

g g z ( x ) = x 2 4 2 x 2 = ( x 2 ) ( x + 2 ) 2 x 2

g ( x ) = 4 x 3 .

At, extreme point, g'(x) = 0

( x + 2 ) ( x 2 ) x 2 = 0

x = -2 or x = 2

Given, that x > 0, hence we have x = 2.

At, x = 2,  g ˙ ( x ) = 4 2 3 = 4 8 = 1 2 > 0

∴x = 2 is a point of local minima and value of local

Minimum is given by g(2) =  2 2 + 2 2 = 1 + 1 = 2 .

(vii)  f ( x ) = 1 x 2 + 2

A(vii)

Given,  f ( x ) = 1 x 2 + 2

g ( x ) = 1 ( x 2 + 2 ) 2 d d x ( x 2 + 2 ) = 2 x ( x 2 + 2 ) 2 .

g ( x ) = [ ( x 2 + 2 ) 2 d d x 2 x 2 x d d x ( x 2 + 2 ) 2 ( x 2 + 2 ) 4 ]

= [ 2 ( x 2 + 2 ) 2 2 x 2 ( x 2 + 2 ) 2 x ] ( x 2 + 2 ) 4 .

= [ 2 ( x 2 + 2 ) 2 [ x 2 + 2 4 x 2 ] ] ( x 2 + 2 ) 4

= 2 ( x 2 + 2 ) ( 2 3 x 2 ) ( x 2 + 2 ) 4 = 2 ( 2 3 x 2 ) ( x 2 + 2 ) 3 .

At extreme points, g'(x) = 0.

2 x ( x 2 + 2 ) 2 = 0 .

 x = 0.

At, x = 0,  g ( x ) = 2 ( 2 2 × 0 2 ) ( 0 2 + 2 ) 3 = 2 ( 2 0 ) 2 3 = 1 2 < 0

∴x = 0 is a point of local maxima and value of local maximum is given by  g ( 0 ) = 1 0 2 + 2 = 1 2 .

Q77. Prove that the following functions do not have maxima or minima:

(i) f(x) = ex

A(i) We have, f(x) = ex

f(x) = ex.

f ( x ) = e x .

At, extreme points,

f(x) = 0

ex = 0 which has no real ‘a’ value

∴f(x) has with maximum or minima

(ii) g(x) = log x

A(ii)

We have, g(x) = log x,

 g(x) =  1 x

At extreme points,

g(x) = 0

1 x = 0 .

 1 = 0 which is not true.

∴g(x) was value minima or maxima

(iii) h(x) = x3 + x2 + x + 1.

A(iii)

We have, h(x) = x3 + x2 + x + 1.

h(x) = 3x2 + 2x + 1

At extreme points,

h(x) = 0

3x2 + 2x + 1 = 0

Q78. Find the absolute maximum value and the absolute minimum value of the following functions in the given intervals:

(i) f(x) =x3 , x ∈ [– 2, 2]

A.(i) We have,

f(x) = x3 , x ∈ [– 2, 2].

f(x) = 3x2.

At, f(x) = 0

3x2 = 0

x = 0 <--[-2, 2].

We shall absolute the value of f at x = 0 and points of interval [ -2, 2]. So,

f(0) = 0

f(- 2) = (- 2)3 = 8

f(2) = 23 = 8.

∴ Absolute maximum value of f(x) = 8 at x = 2

and absolute minimum value of f(x) = -8 at x = -2.

(ii) f (x) = sin x + cos x , x ∈ [0, π]

A.(ii)

We have, f(x) = sin x + cos x , x ∈ [0, π]

f(x) = cos x - sin x.

atf(x) = 0

cosx - sin x = 0

 sinx = cos x

s i n ° x c o r x = 1 t a n x = 1 t a n x = t a n π 4

x = π 4 [ 0 , π ]

(iii) f(x) = 4x  1 2 x 2 , x [ 2 , 9 2 ]

A.(iii)

We have, f(x) = 4x  1 2 x 2 , x [ 2 , 9 2 ]

f(x) = 4 - x

atf(x) = 0

4-  x = 0

 x = 4 [ −2, 9 2 ]

f ( 4 ) = 4 ( 4 ) 1 2 ( 4 ) 2 = 1 6 8 = 8 .

f ( 2 ) = 4 ( 2 ) 1 2 ( 2 ) 2 = 8 2 = 1 0 .

f ( 9 2 ) = 4 ( 9 2 ) 1 2 ( 9 2 ) 2 = 1 8 8 1 8 = 9 4 8 1 8 = 6 3 8 .

= 7.87.5

Hence, absolute maximum value of f(x) = 8 at x = 4

and absolute minimum value of f(x) = -10 at x = -2.

(iv) f(x) = (x - 1)2 + 3, x∈[ -3, 1]

A.(iv)

We have, f(x) = (x - 1)2 + 3, x∈[ -3, 1]

f(x) = 2(x -1)

atf(x) = 0

2(x - 1) = 0

x = 1 [ -3, 1]

∴f(1) = (1 - 1)2 + 3 = 3

f(- 3) = (- 3 - 1)2 + 3 = 16 + 3 = 1

∴ Absolute maximum value of f(x) = 19 at x = -3.

And absolute minimum value of f(x) = 3 at x = 1.

Q79. Find the ma proximumfit that a company can make, if the profit function is given by p(x) = 41 – 72x – 18x2

A.6. We have, p(x) = 41 -f2x - 18x2.

P(x) = - 72 - 36x

P(x) = -36

At extreme point,

- 72 - 36x = 0

  x = 7 2 3 6 = 2  .

At x = - 2, p"(x) = - 36 < 0.

∴x = -2 is a point of local maximum and the value of local

Maximum is given by P(2) = 41 - 72(- 2) - 18(- 2)2

41 + 144 - 72 = 113 units.

Q80. Find both the maximum value and the minimum value of 3x4 – 8x3 + 12x2 – 48x + 25  on the interval [0, 3].

A7. We have, f(x) = 3x4- 8x3 + 12x2- 48x + 25, x ∈ [0, 3].

f(x) = 12x3- 24x2 + 24x - 48.

At f(x) = 0.

12x3- 24x2 + 24x - 48 = 0.

x3- 2x2 + 2x - 4 = 0

x2(x - 2) + 2(x - 2) = 0

(x - 2) + (x2 + 2) = 0

x = 2 ∈ [0, 3] or x = ±√-2  which is not possible as

∴f(x) = 3(2)4- 8(2)3 + 12(2)2- 4(2) + 25.

=48 - 64 + 48 - 96 + 25.

= -39.

f(0) =3(0)4- 8(0)3 + 12(0)2- 48(0) + 25.

= 25.

f(3) = 3(3)4- 8(3)3 + 12(3)2- 48(3) + 25.

= 243 - 216 + 108 - 144 + 25

= 16.

Maximum value of f(x) = 25 at x = 0.

and minimum value of f(x) = -39 at x = 2.

Q81. At what points in the interval [0, 2π], does the function sin 2x attain its maximum value?

A8. We have, f(x) = sin 2x, x ∈ [0, 2π],

f(x) = 2cos 2x.

At f(x) = 0.

2 cos 2x = 0

cos 2x = 0

2 x = ( 2 x + 1 ) π 2 , x = 0 , 1 , 2 , 3 .

x = ( 2 x + 1 ) π 4 .

x = π 4 , 3 π 4 , 5 π 4 , 7 π 4 , [ 0 , 2 π ]

f ( π 4 ) = s i n 2 π 4 = s i n π 2 = 1  .

f ( 3 π 4 ) = s i n 2 ( 3 π 4 ) =   s i n 3 π 2   f ( 7 π 4 )

= s i n ( π + π 2 ) =   = s i n 2 × 7 π 4

= s i n π 2   = s i n 7 π 2

= 1.  = s i n 3 π + π 2

f ( 5 π 4 ) = s i n 2 × ( 5 π 4 ) = s i n 5 π 2 = s i n ( 2 x + π 4 )   = s i n π 2

= s i n π 4 = 1  = 1.

f(0) = sin 2(0) = sin 0 = 0

f(2π) = sin 2(2π) = sin 4π = 0

Hence, the points of maximum  x f x are.

( π 4 , 2 )and ( 5 π 4 , 1 ) .

Q82. What is the maximum value of the function sin x + cos x?

A9. We have, f(x) = sin x + cos x.

f(x) = cos x - sin x.

At f(x) = 0

cosx - sin x = 0

sinx = cos x

s i n x c o s x = 1 .

t a n x = 1 = t a n π 4

x = π 4 o r x = n π + π 4 .

At  x = π 4 + n π  ,

f ( n π + π 4 ) = s i n ( n π + π 4 ) + c o s ( n x + π 4 ) .

= ( 1 ) x s i n π 4 + ( 1 ) n s i n π 4 .

Q83. Find the maximum value of 2x3 – 24x + 107 in the interval [1, 3].

Find the maximum value of the same function in [–3, –1]

A.10. We have, f(x) =2x3- 24x + 107, x[1,3]

f(x) = 6x2- 24.

At f(x) = 0

6x2- 24= 0

x 2 = 2 4 6 = 4

x = ±2. -->x = 2 ∈[1, 3].

So, f(2) = 2(2)3- 24(2) + 107 = 16 - 48 + 107 = 75.

f(1) = 2(1)3- 24(1) + 107 = 2 - 24 + 107 = 85.

f(3) = 2(3)3- 24(3) + 107 = 54 - 72 + 107 = 89

∴ Maximum value of f(x) in interval [1, 3] is 89 at x = 3.

When x ∈  [ -3, -1]

From f(x) = 0

x = -2 ∈ [ -3, -1]

So, f(- 2) = 2(- 2)3- 24(- 2) + 107 = - 16 + 48 + 107 = 139.

f(- 3) = 2(- 3)3- 24(- 3) + 107 = - 54 + 72 + 107 = 125.

f(- 1) = 2(- 1)3- 24(- 1) + 107 = - 2 + 24 + 107 = 129.

∴ Maximum value of f(x) in interval [ -3, -1] is 139 at x = -2.

 

Q84. It is given that at x = 1, the function x4 – 62x2 + ax + 9 attains its maximum value, on the interval [0, 2]. Find the value of a.

A.11. We have, f(x) = x4- 62x2 + ax + 9, x∈ [0, 2].

f(x) = 4x3- 124x + a

∴f(x) active its maxn value at x = 1 [0, 2]

∴f(1) = 0.

4(1)3- 124(1) + a = 0

a = 124 - 4 = 120.

∴a = 120

Q85. Find the maximum and minimum values of x + sin 2x on [0, 2π].

A.12. We have, f(x) = x + sin 2x ,x ∈ [0, 2π].

f(x) = 1 + 2cos 2x

At f(x) = 0

1 + 2 cos2x = 0

c o s 2 x = 1 2 = c o s π 3 = c o s π π 3 = c o s 2 π 3

2 x = 2 n π ± 2 π 3 , n = 1 , 2 , 3 .

x = n π ± 2 π 3

n = 0 , x = ± π 3 x = π 3 [ 0 , 2 π ] .

n = 1 , x = π ± π 3 x = π + π 3 a n d π π 3

= 4 π 3 a d 2 π 3 [ 0 , 2 π ]

n = 2 , x = 2 π ± π 3 x = 2 π + π 3 a d 2 π π 3 .

x = 5 1 3 [ 0 , 2 π ] .

Hence,  x = π 3 , 2 π 3 , 4 π 3 a n d 5 π 3

Missing

At  x = π 3 , f ( π 3 ) = π 3 + s i n 2 π 3 = 1 . 0 5 +   s i n ( π π 3 ) = 1 . 0 5 + s i n π 3

= 1 . 0 5 +√3/2

= 1.05 + 0.87

= 1.92

At  x = 2 π 3 , f ( 2 π 3 ) = 2 π 3 + s i n 2 × 2 π 3   = 2 . 1 0 + s i n ( π + π 3 )

= 2 . 1 0 s i n π 3 = 2 , 1 0 0 . 8 7

= 1.23

At  x = 4 π 3 , f ( 4 π 3 ) = 4 π 3 + s i n 2 × 4 π 3 = 4 2 + s i n ( 3 x π 3 )

= 4 . 2 + s i n π 3 = 4 . 2 + 0 . 8 7 .

=5.07.

At  x = 5 π 3 , f ( 5 π 3 ) = 5 π 3 + s i n 2 × 5 π 3 = 5 . 2 5 + s i n ( 3 x + π 3 )

= 5 2 5 s i n 1 3

= 5.25 - 0.87 = 4.38

At and points,

f(0) = 0 + sin2 × 0 = 0

f(2π) = 2π + sin 2 × 2π = 6.2 + 0 = 6.28

∴Maximum value of f(x) = 6.28 at x = 2π and

minimum value of f(x) = 0 at x= 0

Q86. Find two numbers whose sum is 24 and whose product is as large as possible.

A.13. Let ‘x’ and ‘y’ be the two number

Then, x + y = 24  y = 24 - x

Let ‘P’ be their product then,

P = xy = x (24 - x) = 24x -x2

P(x) = 24x -x2

d p d x = 2 4 2 x .

d 2 p d x 2 = 2 .

At  d p d x = 0

24 - 2x = 0

x = 2 4 2 = 1 2 .

So, P(12)  d 2 p d x 2 = 2 < 0 .

x = 12 is a point of local maxima

Hence, y = 24 - 12 = 12.

The uqdtwno (x, y) is (12, 12).

Q87. Find two positive numbers x and y such that x + y = 60 and xy3 is maximum.

A.14. We have, x + y = 60.where x, y > 0

x = 60 - y.

Let the product P = xy3 = (60 -y) y3 = 60y3-y4

d p d y = 1 8 0 y 2 4 y 3

= 4y2(45 -y)

At  d p d y = 0

4y2(45 -y) = 0

 y = 0 and y = 45

As y > 0, y = 45

When, y > 45,  d p d y = ( + v e ) ( v e )

= ve < 0

Ad y < 45,  d p d y = ( + v e ) ( + v e )

= (+ ve) > 0

∴p is maximum when y = 45 from + ve to- ve or y increases through 45.

So, x = 60 - y = 6Ø - 45 = 15.

Øx = 15 and y = 45.

Q88. Find two positive numbers x and y such that their sum is 35 and the product x2 y5 is a maximum.

A.15. We have, x + y = 35.

y = 35 - x

Let the product, P =x2 y5

P = x2 (35 -x)5

So,  d p d x = x2 5(35 -x)4 (1) + (35 -x)5 2x

= x (35 -x)4[ - 5x + (35 -x) 2]

= x (35 -x)4 [ - 5x + 70 - 2x]

= x (35 -x)4 (70 - 7x)

= 7x (35 -x)4 (10 -x)

At  d p d x = 0

7x (35 -x)4 (10 -x) = 0

 x = 0, 35, 10

As x is a (+) ve number we have only

x = 10, 35

And again (at x = 35) y = 35 = 0 but yis also a (+) ve number

we get, x = 10 (only)

whenx < 10,

d p d x = ( + v e ) ( + v e ) ( + v e ) > 0

and when x > 10,

d p d x = ( + v e ) v e ( + v e ) ( v e ) = ( v e ) < 0

d p d x changes from (+ ve) to ( -ve) as x increases while passing through 10

Hence, x = 10 is a point of local maxima

So, y = 35 - 10 = 25

∴x = 10 and y = 25

Q89. Find two positive numbers whose sum is 16 and the sum of whose cubes is minimum

A.16. Let, x and y be the two positive number

Then, x + y = 16 y = 16 - x

Let p be the sun of the cubes then

p = x3 + y3 = x3 + (16 -x)3 = x3 + (16)3-x3- 48x (16 -x)

p = 163 + 48x2- 76 8x

So,  d p d x = 9 6 x 7 6 8 .

d 2 p d x 2 = 9 6 .

At  d p d x = 0

96x - 768 = 0

x = 7 6 8 9 6 = 8 .

∴at x = 8,  d 2 p d x 2 = 9 6 > 0

So, x = 8 is a point of local minima

So, y = 16 - 8 = 8

Hence, x = 8, y = 8

Q90. A square piece of tin of side 18 cm is to be made into a box without top, by cutting a square from each corner and folding up the flaps to form the box. What should be the side of the square to be cut off so that the volume of the box is the maximum possible.

A.17. Side of the tin square piece = 18 cm

Let x cm be the thought of the square to be cut from each corner.

The volume v of the box after cutting

v = length × breadth × height

= (18 - 2x)(18 - 2xx x

= (18 - 2x)2x

= (324 + 4x2- 72x) x

= 4x3- 72x2 + 324x

So,  d d x = 1 2 x 2 1 4 4 x + 3 2 4

d 2 v d x 2 = 2 4 x 1 4 4

As  d d x = 0 1 2 x 2 1 4 4 x + 3 2 4 = 0

x2- 12x + 27 = 0

x2- 9x- 3x + 27 = 0

x (x - 9) - 3 (x - 9) = 0

(x - 9)(x - 3) = 0

 x = 9 and x = 3

At x = 0, length of box = 18 - 2π9 = 18 - 18 = 0

Which is not possible

And at x = 3,  d 2 y d x 2 = 24(3) - 144 = -72 < 0

∴x = 3 is a point of maximum

Hence, ‘3’ cm (square) is to be cut from each side of the square

So that volume of box is maximum

Q91. A rectangular sheet of tin 45 cm by 24 cm is to be made into a box without top, by cutting off square from each corner and folding up the flaps. What should be the side of the square to be cut off so that the volume of the box is maximum?

A.18. Let ‘x’ cm be the length of side of the square to be cut off from the rectangular surface

Then, the volume v of the box is v = (45 - 2x)(24 - 2x) x

= 1080x - 138x2 + 4x3

So,  d v d x = 1 0 8 0 2 7 6 x + 1 2 x 2

d 2 d x 2 = 2 7 6 + 2 4 x

At,  d v d x = 0 1 0 8 0 2 7 6 x + 1 2 x 2 = 0

x2- 23x + 90 = 0

x2- 5x - 18x + 90 = 0

x(x - 5) - 18 (x - 5) = 0

(x- 5) (x- 18) = 0

 x = 5 and x = 18

At x = 18, breadth = 24 - 2(18) = 24 - 36 = -12 which is not possible

At,  x = 5 , d 2 v d x 2 = 2 7 6 + 2 4 ( 5 ) = 2 7 6 + 1 2 0 = 1 5 0 < 0

Hence, x = 5 is the point of maximum

So, ‘5’ cm length of square seeds to be cut from each corner of the secthgle

Q92. Show that of all the rectangles inscribed in a given fixed circle, the square has the maximum area.

A.19. Let A.B.C.D.be the square increased in a given fixed circle with radius x

Let ‘x’ and ‘y’ be the length and breadth of the rectangle

∴x, y> 0

In ABC, right angle at B,

x2 + y2 = (2x)2

x2 + y2 = 4x2

Q93. Show that the right circular cylinder of given surface and maximum volume is such that its height is equal to the diameter of the base.

A.20. Let r and h be the radius and height of the cylinder

So, r, h > 0

The total surface area s is given by

S = 2πr(h + r) = content .

S 2 π = r ( h + r )  = content = x (say)

h + r = u r

h = k r r = k r 2 r .

Then, the volume v of the cylinder

V = π r 2 h = π r 2 ( k r 2 r ) = π r ( x x 2 ) = π ( k x π 3 )

So,  d v d r = π ( x 3 r 2 )

d 2 d r 2 = 6 π r .

For maximum,  d v d r = 0

π [ x 3 x 2 ) = 0

r 2 = x 3

Q94. Of all the closed cylindrical cans (right circular), of a given volume of 100 cubic centimetres, find the dimensions of the can which has the minimum surface area?

A21. The volume v of a cylinder of height h and radius r is

V = πr2h = 100

h = 1 0 0 π r 2 .

Let, s be the surface area then

S = 2r2hr(r + h) =  2 x ( r 2 + 1 0 0 r π r 2 )

= 2 π ( n 2 + 1 0 0 π n ) .

d S d r = 2 π [ 2 x 1 0 0 1 r 2 ]

d 2 S d r 2 = 2 π [ 2 + 2 0 0 π r 3 ]

At,  d s d r = 2 π [ 2 r 1 0 0 π r 2 ] = 0

2 r = 1 0 0 π r 2

⇒M 3 = 5 0 π M = [ 5 0 π ] 1 3 , > 0 .

At,  x = [ 5 0 π ] 1 3 , d 2S d r 2 = 2 π [ 2 + 2 0 0 π [ 5 0 π ] 1 3 × 3 ]

= 2 π [ 2 + 4 ] = 1 2 π > 0

r = [ 5 0 π ] 1 3  isa point of minimum

And  h = 1 0 0 π [ 5 0 π ] 2 3 = 2 [ 5 0 π ] 1 3 .

Q95. A wire of length 28 m is to be cut into two pieces. One of the pieces is to be made into a square and the other into a circle. What should be the length of the two pieces so that the combined area of the square and the circle is minimum?

A.22. Let x and y in ‘m’ be the length of side of the square the radius of the circle respectily

Then, length of wire = perimeter of square + circumference of circle

28 = 4x + 2πy

 2x + πy = 14

y = 1 4 2 x x

The combine area A of the square and the circle is

A = x2 + πy2

= x 2 + π [ 1 4 2 x π ] 2

= x 2 + [ 2 ( 7 x ) x ] 2

= x 2 + 4 π ( 7 x ) 2 .

So,  dA d x = 2 x + 4 2 π ( 7 x ) + ( 1 ) = 2 x 8 π ( 7 x )

d 2A d x 2 = 2 + 8 π

At,  dA d x = 0

2 x 8 ( 7 x ) π = 0

2 x = 8 π ( 7 x )

2 π x = 5 6 8 x

( 2 x + 8 ) x = 5 6

x = 5 6 2 π + 8 = 2 8 x + 4 .

At,  x = 2 8 π + 4 , d 2A d x 2 = 2 + γ π > 0

x = 2 8 π + 4  isa point of minima

Hence, length of square =  4 x = 4 ( 2 8 1 + 4 ) = 1 1 2 π + 4 m

and length of circle = 2πy

= 2 π ( 1 4 2 x π )

= 2 π [ 1 4 π 2 1 [ 2 8 π + 4 ] ]

= 2 8 4 [ 2 8 π + 4 ] = 2 8 1 1 2 1 + 4

= 2 8 x + 1 1 2 1 1 2 x + 4

= 2 8 π π + 4

Q96. Prove that the volume of the largest cone that can be inscribed in a sphere of radius R is  8 2 7  of the volume of the sphere.

A.23. Let r and h be the radius and height of the one in scribed in the sphere of radius R.

Then, is ΔOBC, rt angle at B (h-r)2 + r2 = R2

h2 + R2- 2hR + h2 = R2

r2 = 2hR -h2

Then the volume v of the cone is,  V = 1 3 × r 2 h   = 1 3 π ( 2 h R h 2 ) h

= 1 3 π ( 2 Rh 2 h 3 ) .

dV d h = π 3 ( 4 Rh 3 h 2 ) .

d 2V d h 2 = π 3 ( 4R 6 h ) .

At  dV d h = 0

π 3 ( 4R h 3 h 2 ) = 0 .

4Rh – 3h2 = 0.

h(4R – 3h) = 0.

h = 0 and  h = 4 R 3

As h> 0,  h = 4 3 .

At  h = 4R 3 , d 2v d h 2 = π 3 [ 4 R 6 × 4rR 3 ]

= 1 3 [ 4R 8 R] = 4 3 < 0

h = 4R 3  is a point of maxima.

and  r 2 = 2 hR h 2 = 2 × 4R 3 R− ( 4R 3 ) 2

= 3 × 8R 2 9 1 6R 2 9

r 2 = 8R 2 9

Hence, Volume of Cone,  = 1 3 π r 2 h

= 1 3 π × 8R 2 9 × 4R 3

= 8 2 7 × 4 3 πR 3 = 8 2 7 ×  Volume of sphere.

Q97. Show that the right circular cone of least curved surface and given volume has an altitude equal to √2  time the radius of the base.

A.24. Let r and h be the radius and height of the cone.

The volume V of the cone is.

= 1 3 π r 2 h

r 2 h = 3V π = k(SAY) r 2 = k h .

And curve surface area S is

Q98. Show that the semi-vertical angle of the cone of the maximum volume and of given slant height is t a n 1 √2.

A.25. Let r, h, l and Ø be the radius, height, slant height and semi-vertical angle respectively of the cone. i.e., r, h, l>0.

Then, Volume V of the cone is

= 1 3 π r 2 h .  

= 1 3 π ( l 2 h 2 ) h

= 1 3 π ( l 2 h h 3 ) .

So,  dV d h = 1 3 π ( l 2 3 h 2 )

d 2V d h 2 = 2 π h

 

 .

Q99. Show that semi-vertical angle of right circular cone of given surface area and maximum volume is s i n 1 1 3  .

A.26.

Choose the correct answer in Questions 27 and 29.

 

Q100. The point on the curve x2 = 2y which is nearest to the point (0, 5) is

(A) (2√2  ,4) (B) (2√2   ,0) (C) (0, 0) (D) (2, 2)

A.27. The equation of the given curve is

x2 = 2y.

Let p(x, y) be a point on the curve.

The distance of p(x, y) from (0, 5) is say S is given by

 

s 2 = x 2 + ( y 5 ) 2

Let z = s2 = x2 + y2 + 25 – 10y = 2y + y2 -10y + 25

z = y2 – 8y + 25

So,  d z d y = 2 y 8

d 2 z d y 2 = 2

At  d z d y = 0 2 y 8 = 0 y = 8 2 = 4

At y = 4,  d 2 z d y 2 = 2 > 0

y = 4, is point of minimum distance.

So, x2 = 2y-->x2 = 2 × 4--> x2 = 8 x = ± 2

Hence, the point of nearest distance are  ( 2√2 , 4 )  and

( 2√2 , 4 ) .

Option (A) is correct.

 

Q101. For all real values of x, the minimum value of 1 x + x 2 1 + x + x 2  is

(A) 0 (B) 1 (C) 3 (D)  1 3

A.28. We have,

f ( x ) = 1 x + x 2 1 + x + x 2 .

f ( x ) = ( 1 + x + x 2 ) d d x ( 1 x + x 2 ) ( 1 x + x 2 ) d d x ( 1 + x + 2 ) ( 1 + x + x 2 ) 2

f ( x ) = ( 1 + x + x 2 ) ( 1 + 2 x ) ( 1 x + x 2 ) ( 1 + 2 x ) ( 1 + x + x 2 ) 2

= 1 + 2 x x + 2 x 2 x 2 + 2 x 3 1 2 x + x + 2 x 2 x 2 2 x 3 ( 1 + x + x 2 ) 2

= 2 + 2 x 2 ( 1 + x + x 2 ) 2 = 2 ( 1 x 2 ) ( 1 + x + x 2 ) 2 = 2 ( 1 + x ) ( 1 x ) ( 1 + x + x 2 ) 2

At f(x) = 0.

2 ( 1 + x ) ( 1 x ) ( 1 + x + x 2 ) 2 = 0

x = 1 and x = -1.

At  x = 1 , f ( 1 ) = 1 1 + 1 1 + 1 + 1 = 1 3   x = 1 , f ( 1 ) = 1 1 + 1 1 + 1 + 1 = 1 3

At x = -1,  f ( 1 ) = 1 + 1 + 1 1 1 + 1 = 3

The maximum value of f(x)  = 1 3 x = 1

Hence, option (D) is correct.

Q102. The maximum value of [ x ( x 1 ) + 1 ] 1 3 , 0

(A)  1 3 1 3  (B)  1 2  (C) 1 (D) 0

A.29. We have,

f ( x ) = [ X(x - 1) +
1]
1 3
, 0 x 1

f ( x ) = 1 3 [ x 2 x + 1 ] 2 3 d d x ( x 2 x + 1 )

= 2 x 1 3 ( x 2 x + 1 ) 2 3

At f(x) = 0.

2x – 1 = 0

x = 1 2 [ 0 , 1 ]

f ( 1 2 ) = [ 1 2 ( 1 2 1 ) + 1 ] 1 3 = [ 1 4 1 2 + 1 ] 1 3 .

= [ 1 2 + 4 4 ] 1 3

[ 3 4 ] 1 3 0 . 9 0

f ( 0 ) = [ 0(0 - 1)
+ 1 ] 1 3 = 1 .

f ( 1 ) = [ 1 (1 - 1) + 1]
1 3
= 1 .

Option (B) is

Hence maximum value of f(x) = at x = 1 and x = 0.

Option (c) is correct.

 

Maths Ncert Solutions class 12th Logo

NCERT Class 12 Application of Derivatives Miscellaneous Exercise Solutions

The Class 12 Application of Derivatives Miscellaneous Exercise contains problems based on a range of concepts covered in the chapter, including the rate of change, increasing and decreasing functions, tangents and normals, approximations, maxima and minima, and optimization problems. This exercise is designed to test students' understanding and ability to apply these concepts in a variety of challenging situations. Miscellaneous Exercise Solutions will have answers to 24 Questions including 14 Long, 4 Short, and 6 MCQs. Students can check the complete solutions of the Chapter 6 Miscellaneous Exercise below;

Application of Derivatives Miscellaneous Exercise Solution

Q103. Using differentials, find the approximate value of each of the following

(a)  ( 1 7 8 1 ) 1 4

A.1. (a) consider y = x 1 4 . L e t , x = 1 6 8 1 & x = 1 8 1 .

T h e n , y = ( x + x ) 1 4 x 1 4 = ( 1 7 8 1 ) 1 4 ( 1 6 8 1 ) 1 4 = ( 1 7 8 1 ) 1 4 2 3 ( 1 7 8 1 ) 1 4 = 2 3 + y

Now,  d y  is approximately equal to  y  and is given by,

d y = ( d y d x ) x = 1 4 ( x ) 3 4 ( x ) ( a s , y = x 1 4 ) = 1 4 ( 1 6 8 1 ) 3 4 ( 1 8 1 ) = 2 7 4 × 8 × 1 8 1 = 1 3 2 × 3 = 1 9 6 = 0 . 0 1 0

Hence, the approximate value of  ( 1 7 8 1 ) 1 4  is  2 3 + 0 . 0 1 0 = 0 . 6 6 7 + 0 . 0 1 0

=0.677

(b)  ( 3 3 ) 1 5

(b) Consider  y = x 1 5 . L e t , x = 3 2 & x = 1

Now,  d y  is approximately equal to  y  and is given by,

d y = ( d y d x ) ( x ) = 1 5 ( x ) 6 5 ( x ) ( a s , y = x 1 5 ) = 1 5 ( x ) 6 ( 1 ) = 1 3 2 0 = 0 . 0 0 3

Hence, the approximate value of 33 1 5
is 1 2 + ( 0 . 0 0 3 = 0 . 4 9 7 )

Q104. Show that the function given by f ( x ) l o g x x ,  has maximum at x = e.

A2. We have, f(x)  l o g x x , x > 0 .

f(x) =  x d d x l o g x l o g x d d x x _ x 2 .

= x × 1 x l o g x x 2 = 1 l o g x x 2

f(x) =  x 2 d d x ( 1 l o g x ) ( 1 l o g x ) d d x x 2 x 4

x 2 ( 1 x ) ( 1 l o g x ) ( 2 x ) x 4

1 2 + 2 l o g x x 3  =  2 l o g x 3 x 3

At extreme points, f(x) = 0.

1 l o g x x 2 = 0 .  

l o g x = 1 = l o g e .

x = e .

At x = e, f"(e) = 2 l o g e 3 e 3 = 2 3 e 3 = 1 e 3 < 0

x = e is a point of maximum.

Q105. The two equal sides of an isosceles triangle with fixed base b are decreasing at the rate of 3 cm per second. How fast is the area decreasing when the two equal sides are equal to the base ?

A3. Let ‘b’ and ‘x’ be the fixed base and equal side of isosceales triangle.

Then,  d x d t = 3 cm/s (Ø decreasing).

Q106. Find the equation of the normal to curve x2 = 4y which passes through the point (1, 2)

A4. Equation of the curve is  y 2 = 4 x . . . . . . . . . . ( i )

2 y d y d x = 4 d y d x = 4 2 y = 2 y d y d x ] ( 1 , 2 ) = 2 2 = 1

Now, the slope of the normal at point  ( 1 , 2 )  is  1 d y d x ] ( 1 , 2 ) = 1 1 = 1

 Equation of the normal at  ( 1 , 2 )  is  y 2 = 1 ( x 1 ) .

y 2 = x + 1 x + y 3 = 0

Q107. Show that the normal at any point θ to the curve x = a cosθ + a θ sin θ, y = a sinθ – aθ cosθ is at a constant distance from the origin.

A5. We have  x = a c o s θ + a θ s i n θ

d x d θ = a s i n θ + a s i n θ + a θ c o s θ = a θ c o s θ y = a s i n θ a θ c o s θ d y d θ = a c o s θ a c o s θ + a θ s i n θ = a θ s i n θ d y d x = d y d θ . d θ d x = a θ s i n θ a θ c o s θ = t a n θ

 Slope of the normal at any point  θ  is  1 t a n θ

The equation of the normal at a given point  ( x , y )  is given by,

y a s i n θ + a θ c o s θ = 1 t a n θ ( x a c o s θ a θ s i n θ ) y s i n θ a s i n 2 θ + a θ s i n θ c o s θ = x c o s θ + a c o s 2 θ + a θ s i n θ c o s θ x c o s θ + y s i n θ a ( s i n 2 θ + c o s 2 θ ) = 0 x c o s θ + y s i n θ a = 0

Now, the perpendicular distance of the normal from the origin is

Q108. Find the intervals in which the function f given by

f(x)=  4 s i n x 2 x x c o s x . 2 + c o s x  is (i) increasing (ii) decreasing

A6. We have, f(x)=  4 s i n x 2 x x c o s x . 2 + c o s x

= 4 s i n x x ( 2 + c o s x ) 2 + c o s x

f ( x ) = 4 s i n x 2 + c o s x x .

So,  f ( x ) = ( 2 + c o s x ) d d x ( 4 s i n x ) 4 s i n x d d x ( 2 + c o s x ) ( 2 + c o s x ) 2 d x d x .

= ( 2 + c o s x ) ( 4 c o s x ) ( 4 s i n x ) ( s i n x ) ( 2 + c o s x ) 2 1 .

= 8 c o s x + 4 c o s 2 x + 4 s i n 2 x ( 2 + c o s x ) 2 1

= 8 c o s x + 4 ( c o s 2 x + s i n 2 x ) ( 2 + c o s x ) 2 1 .

= 8 c o s x + 4 ( 2 + c o s x ) 2 1 .

= 8 c o s x + 4 ( 2 + c o s x ) 2 ( 2 + c o s x ) 2

= 8 c o s x + 4 4 4 c o s x c o s 2 x ( 2 + c o s x ) 2

= 4 c o s x c o s 2 x ( 2 + c o s x ) 2 = c o s x ( 4 c o s x ) ( 2 + c o s x ) 2 .

Now,  ( 2 + c o s x ) 2 > 0 .

And,  4 c o s x > 0 as cos x lies in [1, 1].

So, (i) for increasing, f(x) ≥ 0.

cosx ≥ 0.

x lies in Ist and IVth quadrant.

i.e., f(x) is increasing for  0 x x 2  and  3 x 2 x 2 x .  

(ii) for decreasing, f(x) ≤ 0.

cosx ≤ 0.

x lies in IInd and IIIrd quadrant.

i.e., f(x) is decreasing for  x 2 x 3 x 2  .

Q109. Find the intervals in which the function f given by f ( x ) = x 3 + 1 x 3 , x 0  is

(i) Increasing (ii) Decreasing

A7. We have, f(x) =  x 3 + 1 x 3 , x 0 .

f ( x ) = 3 x 2 3 × 1 x 4 = 3 ( x 2 1 x 4 ) = 3 ( x 6 1 x 4 ) .

= 3 x 4 [ ( x 2 ) 3 1 3 ]

= 3 x 4 ( x 2 1 ) ( x 4 + x 2 + 1 )  {  ( a 3 b 3 ) =   ( a b ) ( a 2 + b 2 + a b )

f ( x ) = 3 ( x 1 ) x 4 ( x + 1 ) ( x 4 + x 2 + 1 ) .   { x 2 b 2 = ( a b )   ( a + b )

At  f ( x ) = 0 .

3 ( x 1 ) ( x + 1 ) ( x 4 + x 2 + 1 ) x 4 = 0

x = 1 x = 1 .   3 ( x 4 + x 2 + 1 ) 0 .  

So we have three disjoint internal i.e.,

( , 1 ] , [ 1 , 1 ] ( 1 , ) .  

When,  x ( , 1 ] .

f ( x ) = ( + ) v e ( v e ) ( v e ) ( + v e ) ( + v e ) = ( + ) v e    and 0    at   x = 1 .  

So, f(x) is increasing.

When    x [ 1 , 1 ]

f ( x ) = ( + v e ) ( v ) ( + v e ) = ( v e )   0 at x = 1and 1

So, f(x) is decreasing.

When  x [ 1 , )

f(x) =    ( + v e ) ( + v e )   ( + v e ) = (   + v e )  on   0 at   x = 1

So, f(x) is increasing.

f(x) is increasing for x (∞,1) and [1, ∞] and decreasing for x [1, 1].

Q110. Find the maximum area of an isosceles triangle inscribed in the ellipse

x 2 a 2 + y 2 b 2 = 1  with its vertex at one end of the major axis.

A.8. The given equation of the ellipse is  x 2 a 2 + y 2 b 2 = 1  (1)

Let the major axis be along x-axis so, vertex is at  ( ± a , 0 )

Let ΔABC be the isosceles triangle inscribed on the

ellipse with one vertex C at (a, 0).

Then, let A have Co-ordinate (x0, yo) from figure.

So, Co-ordinate of B = (x0, y0)

As A and B lies on the ellipse, from equation (i),

Q111. A tank with rectangular base and rectangular sides, open at the top is to be constructed so that its depth is 2 m and volume is 8 m3. If building of tank costs Rs 70 per sq metres for the base and Rs 45 per square metre for sides. What is the cost of least expensive tank?

A.9. Let x and y meters be the length and breath of the rectangular base of the tank respectively.

Then, volume V of the tank is

V = length × depth × breath.

V = 2xy = 8m3(given).

y = 8 2 x = 4 x .

Let ‘t’ be the total cost of building the tank.

Then, t = cost of base + cost of sides.

= 70xy + 45[4x+4y]     {there are four sides.

= 70xy + 180x+ 180y.

= 7 0 x 4 x + 1 8 0 x + 1 8 0 × 4 x .

t = 2 8 0 + 1 8 0 x + 7 2 0 x .

So,  d z d x = 0 + 1 8 0 7 2 0 x 2 .

And  d 2 t d x 2 = 1 4 4 0 x 3

At  d 2 d x = 0 1 8 0 7 2 0 x 2 = 0

x 2 = 7 2 0 1 8 0 = 4

x = ± 2

x = 2, (x = length and it cannot be negative)

At x = 2,  d 2 t d x 2 = 1 4 4 0 2 3 = 1 4 4 0 8 = 1 8 0 > 0 .

x = 2 is point of maxima.

Hence, minimum cost =  2 8 0 + 1 8 0 × 2 + 7 2 0 2  = 280 + 360 + 360 = 1000.

Q112. The sum of the perimeter of a circle and square is k, where k is some constant. Prove that the sum of their areas is least when the side of square is double the radius of the circle.

A.10. Let r and s be the radius of the circle and length of side of square.

Then, sum of perimeter of circle and square = k

2πr +4s = k

s =  k 2 × r 4

The area A be the total areas of the circle and square.

Then, A = πr2 + s2

= π r 2 + ( k 2 π r 4 ) 2 = π r 2 + x 2 + 4 π 2 r 2 4 π r k 1 6

= 1 6 π r 2 + x 2 + 4 π 2 r 2 4 π k r 1 6

= 1 1 6 [ ( 1 6 π + 4 π 2 ) π 2 4 π k r + x 2 ] .

So,  d A d r = 1 1 6 [ ( 1 6 π + 4 x 2 ) 2 n 4 π k . ] .

And  d 2A d r 2 = 1 1 6 [ ( 6 π + 4 π 2 ) 2 ] = 1 6 π + 4 π 2 8

At  d A d r = 0

1 1 6 [ ( 1 6 x + 4 x 2 ) 2 x 4 π k ] = 0

( 1 6 x + 4 x 2 ) 2 x 4 π k = 0

4 π [ 4 + π ] 2 r = 4 π k .

r = k 2 ( 4 + π ) .

At  r = x 2 ( 4 + π ) , d 2A d x 2 = 1 6 π + 4 π 2 8 > 0 .

s =  2 x 2 ( 4 + x ) .

s = 2r.

Hence, proved.

Q113. A window is in the form of a rectangle surmounted by a semicircular opening. The total perimeter of the window is 10 m. Find the dimensions of the window to admit maximum light through the whole opening.

A.11. Let ‘x’ metre be the radius of the semi-circular opening mounded on the length ‘2x’ side of rectangle. Then, let ‘y’ be the breadth of the rectangle.

Then, perimeter of the window = 10m

2 π x 2 + 2 x + y + y = 1 0

x + 2x + 2y + = 10.

y = 1 0 ( π + 2 ) π 2 .

Let the area of the window be A.

Then, A =  1 2 ( π x 2 ) + 2 x y .

= π x 2 2 + 2 x · [ 1 0 ( x + 2 ) x 2 ] .

= π x 2 + 2 0 x 2 π x 2 4 x 2 2

1 2 [-πx2- 4x2 + 20x].

So,  d d x = 1 2 [ -2πx - 8x + 20]

And  d 2 d x 2 = 1 2 [ -2π - 8] = -π -4 = -( π+ 4)

At  d d x = 0 .

1 2 [ -2πx - 8x + 20] = 0

2x + 8x = 20

x =  2 0 2 π + 8  =  1 0 π + 4 .

At x =  1 0 π + 4 , d 2A d x 2 = -( π+ 4) < 0

Øx =  1 0 x + 4  is a point of minima.

And y =  1 0 ( x + 2 ) × ( 1 0 x + 4 ) 2

= 1 0 ( π + 4 ) ( π + 2 ) 1 0 2 ( π + 4 )

= 1 0 π + 4 0 1 0 π 2 0 2 ( x + 4 ) = 1 0 π + 4 .

Ø Dimensions of the window are

length = 2x =  2 0 π + 4 m

breadth = y  1 0 π + 4 m .

radius = y =  1 0 π + 4 m .

Q114. A point on the hypotenuse of a triangle is at distance a and b from the sides of the triangle. Show that the minimum length of the hypotenuse is

( a 2 3 + b 2 3 ) 3 2  .

A.12. Let P be the point on hypotenuse of a triangle. ABC, t angle at B.

Which is at distance a& b from the sides of the triangle.

Let < BAC = < MPC = Ø.

Then, in… ΔANP,

t a n 3 θ = a b

t a n θ = ( a b ) 1 3 .

At tan Ø =  ( a b ) 1 3 ,

d 2 q ˙ d θ 2 = + v e . > 0 .  { Øall trigonometric fxn are + ve in Ist quadrant}.

So, z is least for tanØ =  ( a b ) 1 3 .

As, Sec2Ø = 1 + tan2Ø = 1 +  ( a b ) 2 3 .  =  b 2 3 + a 2 3 b 2 3 .

secØ =  [ b 2 3 + a 2 3 b 2 3 ] 1 2  =  ( a 2 3 + b 2 3 ) 1 2 b 1 3 .

And tan2Ø = 1 c o t 2 θ

cot2Ø = ( a b ) 2 3

And cosec2Ø = 1 + cot2Ø = 1 + ( a b ) 2 3  =  a 2 3 + b 2 3 a 2 3

cosecØ =  ( a 2 3 + b 2 3 ) 1 2 . a 1 3

Q115. Find the points at which the function f given by f (x) = (x – 2)4 (x + 1)3 has (i) local maxima (ii) local minima (iii) point of inflexion

A.13. We have,

f(x) = (x- 2)4 (x + 1)3.

So, f(x) = (x- 2)4. 3(x + 1)2 + (x + 1)3. 4(x- 2)3.

= (x- 2)3 [x + 1)2 [3 (x- 2) + 4(x + 1)]

= (x- 2)3 (x + 1)2 (3x- 6 + 4x + 4)

= (x- 2)3 (x + 1)2 (7x- 2).

At f(x) = 0.

(x- 2)3 (x + 1)2. (7x- 2) = 0.

x = 2, x = -1 or x = 2 7 .

As (x + 1)2> 0,we shave evaluate for the remaining factor.

At x = 2,

When x< 2, f(x) = ( -ve)(+ ve)(+ ve) = ( -ve) < 0.

When x> 2, f(x) = (+ ve)(+ ve)(+ ve) = (+ ve) > 0.

Øf(x) change from ( -ve) to (+ ve) as x increases

So, x = 2 is a point of local minima

At x = -1.

When x< -1, f(x) = ( -ve)(+ ve)( -ve) =∉ , ve > 0.

When x> -1, f(x) = ( -ve)(+ ve)(+ ve) =∉ , ve > 0.

So, f(x) does not change through x ---1.

Hence, x = -1 is a point of infixion

At x =  2 7 ,

When x<  2 7 , f(x) = ( -ve)(+ ve)( -ve) = ( -ve > 0.

When x<  2 7 , f(x) = ( -ve)(+ ve)(+ ve) = ( -ve < 0.

f(x) change from (+ ve) to ( -ve) as x increases through 2 7 .

x =  2 7  is a point of local maxima

Q116. Find the absolute maximum and minimum values of the function f given by f (x) =cos2 x + sin x, x ∈ [0, π]

A.14. We have,

f(x) = cos2x + sin x, x∈ [0,π ].

So, f(x) = 2 cos x ( -sin x) + cos x = cos x (1 - 2 sin x).

At f(x) = 0

cosx (1 - 2 sin x) = 0

cosx = 0  or  1 - 2 sin x = 0

cosx = cos  π 2  or sin x =  1 2  = sin  π 6  = sin  x π 6

x=  π 2  , x =  π 6  and x =  5 π 6  [0,π ].

So, f  ( π 2 ) = cos2 π 2  + sin  π 2  = 1.

Absolute minimum of f(x) =  5 4  and absolute minimum of f(x) = 1.

Q117. Show that the altitude of the right circular cone of maximum volume that can be inscribed in a sphere of radius r is 4 r 3  .

A.15. Let x be the radius of the sphere ad x be radius of the right circular cone.

Let height of cone = y

Then, in ΔOBA,

(y-r)2 + x2 = r2

y2 + r2- 2ry + x2 = r2

x2=2ry - y2

So, the volume V of the cone is

V= 1 3 π · x 2 · y   { = 1 3 π ( r a d i u s ) 2 ×hight }

= 1 3 × ( 2 x y y 2 ) y

= 1 3 x ( 2 x y 2 y 3 ) .

So,  d v d y = π 3 [ 4 x y 3 y 2 ]

And  d 2 d y 2 = x 3 [ 4 x 6 y ] .

At  d V d y = 0 .

π 3 [ 4 x y 3 y 2 ] = 0

4x -y- 3y2 = 0

y = 4 x 3  asy> 0.

At y =  4 π 3 , d 2 v d y 2 = π 3 [ 4 x 8 x ]  =  4 π r 3 < 0 .

Ø V is maximum when y = 4 π 3

Q.118. Let f be a function defined on [a, b] such that f ′(x) > 0, for all x ∈ (a, b).

Then prove that f is an increasing function on (a, b).

A.16. We have,

f(x) defined on [a, b]

And f(x) > 0 ∀ x ∈ [a, b].

Let x1, x2 ∈ [a, b] and x2>x1

In the internal x1, x2], f(x) will also be continuous and differentiable.

Hence by mean value theorem, there exist c [x1, x2] such that

f ( c ) = f ( x 2 ) f ( x 1 ) x 2 x 1 .

f(x) > 0 ∀ x ∈ [a, b].

Then, f(c) > 0.

f ( x 2 ) f ( x 1 ) x 2 x 1 > 0

i.e., f(x2) -f(x1) > 0

f(x2) >f(x1).

Hence, the function f(x) is always increasing on [a, b]

Q119. Show that the height of the cylinder of maximum volume that can be inscribed in a sphere of radius R is 2R/√3. Also find the maximum volume.

Q120. Show that height of the cylinder of greatest volume which can be inscribed in a right circular cone of height h and semi vertical angle α is one-third that of the cone and the greatest volume of cylinder is 4 2 7 π h 3 t a n 2 α  .

A.18.

Choose the correct answer in the questions from 19 to 24.

 

Q121. A cylindrical tank of radius 10 m is being filled with wheat at the rate of 314 cubic metre per hour. Then the depth of the wheat is increasing at the rate of (A) 1 m/h (B) 0.1 m/h (C) 1.1 m/h (D) 0.5 m/h

A.19. Let x be the depth of the wheat inside the cylindrical tank is with radius = 10 cm

Then, Volume V of the cylindrical tank is

V = π(10)2x = 100πpx m3

As  d d t = 3 1 4 3 h

1 0 0 π d x d t = 3 1 4

d x d t = 3 1 4 1 0 0 π = 3 1 4 1 0 0 π 3 . 1 4 = 3 1 4 3 1 4 = 1 . m h  i e, rate of increasing of depth

option (A) is correct

Q122. The line y = mx + 1 is a tangent to the curve y2 = 4x if the value of m is

(A) 1 (B) 2 (C) 3 (D)  1 2

A.21. The equation of the tangent to the given curve is  y = m x + 1 .

Now, substituting  y = m x + 1 .  in  y 2 = 4 x ,  we get:

( m x + 1 ) 2 = 4 x m 2 x 2 + 1 + 2 m x 4 x = 0 m 2 x 2 + x ( 2 m 4 ) + 1 = 0 . . . . . . . . . . ( i )

Since a tangent touches the curve at one point, the roots of equation (i) must be equal.

Therefore, we have:

Discriminant = 0

( 2 m 4 ) 2 4 ( m ) 2 ( 1 ) = 0 4 m 2 + 1 6 1 6 m 4 m 2 = 0 1 6 1 6 m = 0 m = 1

Hence, the required value of m is 1.

Therefore, option (A) is correct.

Q123. The normal at the point (1,1) on the curve 2y + x2 = 3 is (A) x + y = 0 (B) x – y = 0 (C) x + y +1 = 0 (D) x – y = 1

A.22. The equation of the given curve is  2 y + x 2 = 3  .

Differentiate with respect to x, we have:

2 d y d x + 2 x = 0 d y d x = x d y d x ] ( 1 , 1 ) = 1

The slope of the normal to the given curve at point (1,1) is

1 d y d x ] ( 1 , 1 ) = 1

Hence, the equation of the normal to the given curve at (1,1) is given as:

y 1 = 1 ( x 1 ) y 1 = x 1 x y = 0

Therefore, option (B) is correct.

Q124. The normal to the curve x2 = 4y passing (1,2) is (A) x + y = 3 (B) x – y = 3 (C) x + y = 1 (D) x – y = 1

A.23. The equation of the given curve is  x 2 = 4 y

Differentiating with respect to x, we have:

2 x = 4 . d y d x d y d x = x 2

The slope of the normal to the given curve at point (h,k) is given by,

1 d y d x ] ( h , k ) = 2 h

 Equation of the normal at point (h,k) is given as:

y k = 2 h ( x h )

Now, it is given that the normal passes through the point (1,2).

Therefore, we have:

2 k = 2 h ( 1 h ) o r , k = 2 + 2 h ( 1 h ) . . . . . . . . . . ( i )

Since (h,k) lies on the curve  x 2 = 4 y  ,we have  h 2 = 4 k

k = h 2 4

From equation (i), we have:

h 2 4 = 2 + 2 h ( 1 h ) h 3 4 = 2 h + 2 2 h = 2

h 3 = 8 h = 2 k = h 2 4 k = 1

Hence, the equation of the normal is given as:

y 1 = 2 2 ( x 2 ) y 1 = ( x 2 ) x + y = 3

Therefore, option (A) is correct.

Q125. The points on the curve 9y2 = x3 , where the normal to the curve makes equal intercepts with the axes are

(A)  ( 4 , ± 8 3 )  (B)  4 , 8 3  (C)  4 , ± 3 8  (D)  ± 4 , 3 8

A.24. The equation of the given curve is  9 y 2 = x 3

Differentiate with respect to x, we have:

9 ( 2 y ) d y d x = 3 x 2 d y d x = x 2 6 y

The slope of the normal to the given curve at point  ( x 1 , y 1 )  is

1 d y d x ] ( x 1 , y 1 ) = 6 y 1 x 1 2

 The equation of the normal to the curve at  ( x 1 , y 1 )  is

y y 1 = 6 y 1 x 1 2 ( x x 1 ) x 1 2 y x 1 2 y 1 = 6 x y 1 + 6 x 1 y 1 6 x y 1 + x 1 2 y = 6 x 1 y 1 + x 1 2 y 1 6 x y 1 6 x 1 y 1 + x 1 2 y 1 + x 1 2 y 6 x 1 y 1 + x 1 2 y 1 = 1 x x 1 ( 6 + x 1 ) 6 + y y 1 ( 6 + x 1 ) x 1

It is given that the normal makes intercepts with the axes.

Therefore, we have:

x 1 ( 6 + x 1 ) 6 = y 1 ( 6 + x 1 ) x 1 x 1 6 = y 1 x 1 x 1 2 = 6 y 1 . . . . . . . . . . ( i )

Also, the point  ( x 1 , y 1 )  lies on the curve, so we have

9 y 1 2 = x 1 3 . . . . . . . . . . ( i i )

From (i) and (ii), we have:

9 ( x 1 2 6 ) = x 1 3 x 1 4 4 = x 1 3 x 1 = 4

From (ii), we have:

9 y 1 2 = ( 4 ) 3 = 6 4 y 1 2 = 6 4 9 y 1 = ± 8 3

Hence, the required points are  ( 4 , ± 8 3 )  .

Therefore, option (A) is correct.

 

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Maths Ncert Solutions class 12th Exam

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