Class 12th

Let S be a non-empty set and P (S) be its power set. Let any two subsets A and B of S.

It is given that: $P\left(X\right)xP\left(X\right)\to P\left(X\right)$ is defined as $A.B=A\cap B\forall A,B\in P\left(X\right)$

We know that $A\cap X=A=X\cap A\forall A\in P\left(X\right)$

$\Rightarrow A.X=A=X.A\forall A\in P\left(X\right)$

Thus, X is the identity element for the given binary operation*.

Now, an element is  $A\in P\left(X\right)$ invertible if there exists $B\in P\left(X\right)$ such that

$A*B=X=B*A$  (As X is the identity element)

i.e.

$A\cap B=X=B\cap A$

This case is possible only when $A=X=B.$

Thus, X is the only invertible element in P (X) with respect to the given operation*.

Hence, the given result is proved.

Since every set is a subset of itself, ARA for all A ∈ P (X).

∴R is reflexive.

Let ARB ⇒ A ⊂ B.

This cannot be implied to B ⊂ A.

For instance, if A = {1, 2} and B = {1, 2, 3}, then it cannot be implied that B is related to A.

∴ R is not symmetric.

Further, if ARB and BRC, then A ⊂ B and B ⊂ C.

⇒ A ⊂ C

⇒ ARC 

∴ R is transitive.

Hence, R is not an equivalence relation since it is not symmetric.

Define $f:{\rm N}\to {\rm N}$ by

$f\left(x\right)=x+1$

And,  $g:{\rm N}\to {\rm N}$ by,

$g\left(x\right)\left\{\begin{array}{cc}\hfill x-1\hfill & \hfill if,x>1\hfill \\ \hfill 1\hfill & \hfill f,x=1\hfill \end{array}\right\}$

We first show that g is not onto.

For this, consider element 1 in co-domain N. it is clear that this element is not an image of any of the elements in domain.

$\therefore f$ is not onto.

Now,  $gof:{\rm N}\to {\rm N}$ is defined by,

$\begin{array}{l}gof\left(x\right)=g\left(f\left(x\right)\right)=g\left(x+1\right)=\left(x+1\right)-1\left[x,in{\rm N}=>\left(x+1\right)>1\right]\\ =x\end{array}$

Then, it is clear that for $y\in {\rm N}$ , there exists $x=y\in {\rm N}$ such that $gof\left(x\right)=gof\left(y\right)$

Hence, gof is onto.

Define $f:N\to Z$ as $f\left(x\right)$ and $g:Z\to Z$ as $g\left(x\right)=\left|x\right|$

We first show that g is not injective.

It can be observed that:

$\begin{array}{l}g\left(-1\right)=\left|-1\right|=1\\ g\left(1\right)=\left|1\right|=1\\ \therefore g\left(-1\right)=g\left(1\right),but,-1\ne 1\end{array}$

$\therefore g$ is not injective.

Now, $gof:N\to Z$ is defined as

$gof\left(x\right)=y\left(f\left(x\right)\right)=y\left(x\right)=\left|x\right|$

Let $x,y\in N$ such that $gof\left(x\right)-gof\left(y\right)$

$\Rightarrow \left|x\right|=\left|y\right|$

Since $x,y\in N$ , both are positive.

$\therefore \left|x\right|=\left|y\right|\Rightarrow x=y$

Hence, gof is injective

f: R → R is given as f (x) = x³.

Suppose f (x) = f (y), where x, y ∈ R.

⇒ x³ = y³  … (1)

Now, we need to show that x = y.

Suppose x ≠ y, their cubes will also not be equal.

⇒ x³ ≠ y³

However, this will be a contradiction to (1).

∴ x = y

Hence, f is injective.

It is given that $f:R\to R$ is defined as $f\left(x\right)=x^2-3x+2.$

$\begin{array}{l}f\left(f\left(x\right)\right)=f\left(x^2-3x+2\right)\\ ={\left(x^2+3x+2\right)}^2-3\left(x^2-3x+2\right)+2\\ =x^4+9x^2+4-6x^2-12x+4x^2-3x^2+9x-6+2\\ =x^4-6x^2+10x^2-3x\end{array}$

It is given that $f:R\to R$ is defined as $f\left(x\right)=10x+7.$

One-one:

$\begin{array}{l}Let,f\left(x\right)=f\left(y\right),where,x,y\in R\\ \Rightarrow 10x+7=10y+7\\ \Rightarrow x=y\end{array}$

$\therefore f$ is a one-one function.

Onto:

$\begin{array}{l}For,y\in R,let,y=10x+7\\ \Rightarrow x=\frac{y-7}{10}\in R\end{array}$

Therefore, for any $y\in R$ ,there exists $x=\frac{y-7}{10}\in R$

Such that

$f\left(x\right)=f\left(\frac{y-7}{10}\right)=10\left(\frac{y-7}{10}\right)+7=y-7+7=y$

$\therefore f$ is onto.

Therefore, f is one-one and onto.

Thus, f is an invertible function.

Let us define $g:R\to R$ as $g\left(y\right)=\frac{y-7}{10}$

Now, we have

$\begin{array}{l}gof\left(x\right)=g\left(f\left(x\right)\right)=g\left(10x+7\right)=\frac{\left(10x+7\right)-7}{10}=10\frac{x}{10}=10\\ And\\ fog\left(y\right)=f\left(g\left(y\right)\right)=f\left(\frac{y-7}{10}\right)=10\left(\frac{y-7}{10}\right)+7=y-7+7=y\end{array}$

Hence, the required function $g:R\to R$ is defined as $g\left(y\right)=\frac{y-7}{10}$

On N, the operation * is defined as a * b = a³ + b³.

For, a, b, ∈ N, we have:

a * b =  a³ + b³ = b³ + a³  = b * a [Addition is commutative in N]

Therefore, the operation * is commutative.

It can be observed that:

  (1*2)*3 = (1³+2³)*3 = 9 * 3 = 93 + 33 = 729 + 27 = 756

Also, 1* (2*3) = 1* (2³ +3³) = 1* (8 +27) = 1 * 35

= 1³ +35³ = 1 + (35)³ = 1 + 42875 = 42876.

∴ (1 * 2) * 3 ≠ 1 * (2 * 3) ; where 1, 2, 3 ∈ N

Therefore, the operation * is not associative.

Hence, the operation * is commutative, but not associative. Thus, the correct answer is B.