Ncert Solutions Maths class 12th

121. Kindly go through the solution

In (D), each of the terms has a degree 2.

Hence, (D) is homogenous

$\therefore$ Option (D) is correct.

120. Let $y={\left(5x\right)}^{3cos2x}$

Taking log,

$logy=3cos2x\left[log5x\right]$

Differentiating w r t. x,

$\frac{1}{y}\frac{dy}{dx}=3cos2x\frac{d}{dx}log5x+3log5x\frac{d}{dx}cos2x$

$=3\left[cos2x\cdot \frac{1}{5x}\frac{d}{dx}\left(5x\right)-log5x*sin2x\frac{d}{dx}2x\right]$

$\frac{dy}{dx}=3y\left[cos2x*\frac{5}{5x}-log5x\cdot sin2x\cdot 2\right]$

$=3{\left(5x\right)}^{3cos2x}\left[\frac{cos2x}{x}-2sin2xlog5x\right]$

For a homogenous D.E. of the formula $f\left(\frac{y}{x}\right)$

We put,  $\frac{x}{y}=0=x=vy$

$\therefore$ Option (c) is correct.

The given D.E. is

$\begin{array}{l}2xy+y^2-2x^2\frac{dy}{dx}=0\\ =2x^2\frac{dy}{dx}=2xy+y^2\\ =\frac{dy}{dx}=\frac{2xy+y^2}{2x^2}=\frac{y}{x}+\frac{1}{2}{\left(\frac{y}{x}\right)}^2=f\left(\frac{y}{x}\right)\end{array}$

i.e, the given is homogenous.

Let, $y=vx$ $=\frac{y}{x}=v$ so that $\frac{dy}{dx}=v+x\frac{dv}{dx}$ is the D.E.

Then, $v+x\frac{dv}{dx}=v+\frac{1}{2}{v}^2$

$\begin{array}{l}=x\frac{dv}{dx}=\frac{1}{2}{v}^2\\ =\frac{dv}{v^2}=\frac{dx}{2x}\end{array}$

Now, $={\displaystyle \int \frac{dv}{v^2}={\displaystyle \int \frac{dx}{2x}}}$

$\begin{array}{l}=\frac{v-2+1}{-2+1}=\frac{1}{2}log\left|x\right|+c\\ =\frac{-1}{v}=\frac{1}{2}log\left|x\right|+c\end{array}$

Putting back $\frac{y}{x}=v$ we get,

$=-\frac{x}{y}=\frac{1}{2}log\left|x\right|+c$

$Given\frac{y}{x}=vwhenx=1$ and y= 2

$\begin{array}{l}=-\frac{1}{2}=\frac{1}{2}log\left|1\right|+c\\ =c=-\frac{1}{2}\end{array}$

$\therefore$ The particular solution is,

$\begin{array}{l}=-\frac{x}{y}=\frac{1}{2}log\left|x\right|-\frac{1}{2}\\ =-\frac{2x}{y}=log\left|x\right|-1\\ =y=\frac{-2x}{log\left|x\right|-1}=\frac{2x}{1-log\left|x\right|}\end{array}$

119. Let $y=si{n}^{3}x+co{s}^{6}x$

So,  $\frac{dy}{dx}=\frac{d}{dx}\left(si{n}^{3}x+co{s}^{6}x\right)$

$=3si{n}^{2}x\frac{d}{dx}sinx+6co{s}^{5}x\frac{d}{dx}cosx$

$=3si{n}^{2}xcosx-6co{s}^{5}xsinx$

$=3sinxcosx\left(sinx-2co{s}^4\right)$

The given D.E.is

$\begin{array}{l}\frac{dy}{dx}-\frac{y}{x}+cosec\left(\frac{y}{x}\right)=0\\ =\frac{dy}{dx}=\frac{y}{x}-cosec\left(\frac{y}{x}\right)=f\left(\frac{y}{x}\right)\end{array}$

i.e, the given D.E. is homogenous.

Let, $y=vx=\frac{y}{x}=v$ So that, $\frac{dy}{dx}=v+x\frac{dv}{dx}$ in the D.E

Then, $v+x\frac{dv}{dx}=v-cosecv$

$\begin{array}{l}=x\frac{dv}{dx}=-cosecv\\ =\frac{dv}{cosecv}=-\frac{dx}{x}\\ =sinvdv=-\frac{dx}{x}\end{array}$

Integrating both sides we get,

$\begin{array}{l}{\displaystyle \int sinvdv=-{\displaystyle \int \frac{dx}{x}}}\\ =-cosv=-log\left|x\right|+c\\ =cosv=log\left|x\right|-c\end{array}$

Putting back $v=\frac{y}{x}$ we get,

$=cos\frac{y}{x}=log\left|x\right|-c$

Given, $y=0,when,x=1$

$\begin{array}{l}=cos0=log1-c\\ =c=-1\end{array}$

$\therefore$ The required particular solution is

$\begin{array}{l}cos\left(\frac{y}{x}\right)=log\left|x\right|+1=log\left|x\right|+log\left|c\right|\\ =cos\left(\frac{y}{x}\right)=log\left|cx\right|\end{array}$

118. Let $y={\left(3x^2-9x+5\right)}^9$

So,  $\frac{dy}{dx}=9{\left(3x^2-9x+5\right)}^8\frac{d}{dx}\left(3x^2-9x+5\right)$

$=9{\left(3x^2-9x+5\right)}^8*\left(6x-9\right)$

$=27{\left(3x^2-9x+5\right)}^8\left(2x-3\right)$

The given D.E.is

$\begin{array}{l}\left[xsi{n}^2\left(\frac{y}{x}\right)-y\right]dx+xdy=0\\ =\left[xsi{n}^2\left(\frac{y}{x}\right)-y\right]dx=-xdy\\ =\frac{dy}{dx}=\frac{\left[xsi{n}^2\left(\frac{y}{x}\right)-y\right]}{-x}\\ =-\left[si{n}^2\left(\frac{y}{x}\right)-\frac{y}{x}\right]=f\left(\frac{y}{x}\right)\end{array}$

i.e, the given D.E is homogenous.

Let, $y=vx=\frac{y}{x}=v$ so that, $\frac{dy}{dx}=vx\frac{dv}{dx}$ in the D.E.

$\begin{array}{l}=v+\frac{dv}{dx}=-\left[si{n}^{2}v-v\right]=v-si{n}^{2}v\\ =\frac{dv}{dx}=-si{n}^{2}v\\ =\frac{dv}{si{n}^{2}v}=-dx\end{array}$

Integrating both sides we get,

$\begin{array}{l}={\displaystyle \int cose{c}^{2}vdv={\displaystyle \int -dx}}\\ =-cotv=-log\left|x\right|+c\\ =cotv=log\left|x\right|-c\end{array}$

Putting back $v=\frac{y}{x}$ we have,

$cot\frac{y}{x}=log\left|x\right|-c$

Then, $y=\frac{\pi }{4}$ when, $x=1$

$\begin{array}{l}cot\frac{\pi }{4}=log\left|1\right|-c\\ =c=-1\end{array}$

$\therefore$ The required particular solution is,

$\begin{array}{l}cot\frac{y}{x}=log\left|x\right|+1=log\left|x\right|+logc\left\{?loge=1\right\}\\ =cot\frac{y}{x}=log\left|ex\right|\end{array}$

The given D.E. is

. $\begin{array}{l}{x}^{2}dy+\left(xy+y^2\right)dx=0\\ =x^{2}dy=-\left(xy+y^2\right)dx\\ =\frac{dy}{dx}=-\left(\frac{xy+y^2}{x^2}\right)=-\left[\frac{y}{x}+\left(\frac{y^2}{x}\right)\right]=f\left(\frac{y}{x}\right)\end{array}$

i.e, the D.E is homogenous.

Let, $y=vx=v=\frac{y}{x}$ so that $\frac{dy}{dx}=v+x\frac{dv}{dx}$ in the given D.E.

Then, $v+x\frac{dv}{dx}=-\left[v+v^2\right]=-v-v^2$

$\begin{array}{l}=x\frac{dv}{dx}=-2v-v^2=-v\left(2+v\right)\\ =\frac{dv}{v\left(2+v\right)}=-\frac{dx}{x}\end{array}$

Integrating both sides we get,

$\begin{array}{l}{\displaystyle \int \frac{dv}{v\left(2+v\right)}=-{\displaystyle \int \frac{dx}{x}}}\\ =\frac{1}{2}{\displaystyle \int \frac{2dv}{2\left(v+2\right)}=}-{\displaystyle \int \frac{dx}{x}}\\ =\frac{1}{2}{\displaystyle \int \frac{v+2-v}{v\left(v+2\right)}}dv={\displaystyle \int -\frac{dx}{x}}\\ =\frac{1}{2}\left\{{\displaystyle \int \frac{1}{v}dv-\frac{1}{v+2}dv}\right\}={\displaystyle \int -\frac{dx}{x}}\end{array}$

$\begin{array}{l}=\frac{1}{2}\left[logv-log\left|v+2\right|\right]=-logx+logc\\ =\frac{1}{2}log\left(\frac{v}{v+2}\right)=log\frac{c}{x}\\ =log{\left(\frac{v}{v+2}\right)}^{\frac{1}{2}}=log\frac{c}{x}\\ ={\left(\frac{v}{v+2}\right)}^{\frac{1}{2}}=\frac{c}{x}\\ =\frac{v}{v+2}={\left(\frac{c}{x}\right)}^2\end{array}$

Putting back $v=\frac{y}{x}$ we get,

$\begin{array}{l}=\frac{\frac{y}{x}}{\frac{y}{x}+2}={\left(\frac{c}{x}\right)}^2\\ =\frac{y}{y+2x}={\left(\frac{c}{x}\right)}^2\\ =\frac{x^{2}y}{y+2x}=c^2\end{array}$

Given, y = 1 when x = 1

So, $=\frac{1}{1+2}=c^2=c^2=\frac{1}{3}$

Hence, the required particular solution is,

$\begin{array}{l}=\frac{x^{2}y}{y+2x}=\frac{1}{3}\\ =3x^{2}y=y+2x\end{array}$