# Relations and Functions

## Class 12 NCERT

### NCERT

1   Determine Whether each of the following relations are reflexive, symmetric and transitive:$\\$ (i) Relation $R$ in the set $A = {1, 2, 3...13, 14}$ defined$\\$ $R =\{(x, y) \colon 3 x - y = 0\}$$\\ (ii) Relation R in the set N of natural numbers defined as\\ R= \{(x, y) \colon y = x + 5 \ and \ x < 4\}$$\\$ (iii) Relation $R$ in the set $A = (1, 2, 3, 4, 5, 6)$ as$\\$ $R= \{(x, y): y$ is divisible by $x\}$$\\ (iv) Relation R in the set Z of all integers defined as\\ R = \{(x, y): x - y is as integer\}$$\\$ (v) Relation $R$ in the set $A$ of human beings in a town at a particular time given by$\\$ $(a) R = \{(x, y): x \ and \ y \ work\ at\ the \ same \ place\}$$\\ (b) R = \{(x, y): x and y live in the same locality\}$$\\$ $(c) R = \{(x, y): x$ is exactly $7 cm$ taller than $y \}$$\\ (d) R =\{(x, y): x is wife of y\}$$\\$ $(e) R = \{(x, y): x$ is father of $y\}$

##### Solution :

(v) (a) R = $\{(x, y): x$ and y work at the same place$\}$$\\ \implies (x, x) \in R \ \ \ [as x and x work at the same place]\\ \therefore R is reflexive.\\ If (x, y) \in R, then x and y work at the same place. \implies y and x work at the same place.\\ \implies (y, x) \in R.$$\\$$\therefore R is symmetric.\\ Now, let (x, y), (y, z) \in R\\ \implies x and y work at the same place and y and z work at the same place.\\ \implies x and z work at the same place.\\ \implies (x, z) \in R\\ \therefore R is transitive.\\ Hence, R is reflexive, symmetric and transitive. (d) R = \{(x, y): x is the wife of y\} Now, (x, x) \notin R\\ Since x cannot be the wife of herself. \therefore R is not reflexive.\\ Now, let (x, y) \in R\\ implies x is the wife of y.\\ Clearly y is not the wife of x. \therefore (y, x) \notin R$$\\$ Indeed, if x is the wife of y, then y is the husband of x. $\therefore$ R is not transitive.$\\$ Let (x, y), (y, z) $\in$ R $\\$ $\implies$ x is the wife of y and y is the wife of z.$\\$ This case is not possible. Also, this does not imply that x is the wife of z.$\\$ $\therefore (x, z) \notin R$$\\ \therefore R is not transitive.\\ Hence, R is neither reflexive, nor symmetric, nor transitive. (ii) R = \{(x, y): y = x + 5 and x < 4\} = \{(1, 6), (2, 7), (3, 8)\}$$\\$ It is clear that $(1, 1) \notin R.\\ \therefore R$ is not reflexive.$\\$ $(1, 6) \in R$ But, $(1, 6) \notin R.\\ \therefore R$ is not symmetric.$\\$ Now, since there is no pair in $R$ such that $(x, y)$ and $(y, z) \in R$, then $(x, z)$ cannot belong to $R$.$\\$ $\therefore R$ is not transitive.$\\$ Hence, $R$ is neither reflexive, nor symmetric, nor transitive.

(c) R = $\{(x, y):$ x is exactly 7 cm taller than y$\}$$\\ Now, (x, x) \in R\\ Since human being x cannot be taller than himself. \therefore R is not reflexive.\\ Now, let (x, y) \in R.\\ \implies x is exactly 7 cm taller than y.\\ Then, y is not taller than x. [Since, y is 7 cm smaller than x]\\ \therefore (y, x) \notin R$$\\$ Indeed if x is exactly $7 cm$ taller than y, then y is exactly $7 cm$ shorter than x.$\\$ $\therefore$ R is not symmetric.$\\$ Now, Let (x, y), (y, z) $\in$ R.$\\$ $\implies$ x is exactly $7 cm$ taller than y and y is exactly $7 cm$ taller than z.$\\$ $\implies$ x is exactly $14 cm$ taller than z .$\\$ $\therefore (x, z) \notin R\\ \therefore R$ is not transitive.$\\$ Hence, R is neither reflexive, nor symmetric, nor transitive.

$(iv) R = \{(x, y): x - y$ is an integer$\} $$\\ Now, for every x \in Z, (x, x) \in R as x - x = 0 is an integer.\\ \therefore R is reflexive.\\ Now, for every x, y \in Z, if (x, y) \in R, then x - y is an integer.\\ \implies -(x - y) is also an integer.\\ \implies (y - x) is an integer.\\ \therefore (y, x) \in R\\ \therefore R is symmetric.\\ Now, Let (x, y) and (y, z) \in R, where x, y, z \in Z. \implies (x - y) and (y - z) are integers.\\ \implies x - z = (x - y) + (y - z) is an integer.\\$$ \therefore (x, z) \in R\\ \therefore R$ is transitive.$\\$ Hence, R is reflexive, symmetric, and transitive.

$(i) A = \{1, 2, 3 ... 13, 14\} \\ R = \{(x, y): 3x - y = 0\}\\ \therefore R = \{(1, 3), (2, 6), (3, 9), (4, 12)\}$$\\ R is not reflexive since (1, 1), (2, 2) ... (14, 14) \notin R.$$\\$ Also, $R$ is not symmetric as $(1, 3) \in R$, but $(3, 1) \notin R. [3(3) - 1 \neq 0]$$\\ Also, R is not transitive as (1, 3), (3, 9) \in R, but (1, 9) \notin R. [3(1) 9 \neq 0]$$\\$ Hence, $R$ is neither reflexive, nor symmetric, nor transitive.

(b) R = $\{(x, y):$ x and y live in the same locality$\}$$\\ Clearly, (x, x) \in R as x and x is the same human being.\\ \therefore R is reflexive.\\ If (x, y) \in R, then x and y live in the same locality.\\ \implies y and x live in the same locality.\\ \implies (y, x) \in R\\ \therefore R is symmetric.\\ Now, let (x, y) \in R and (y, z) \in R.\\ \implies x and y live in the same locality and y and z live in the same locality.\\ \implies x and z live in the same locality.\\ \implies (x, z) \in R$$\\$$\therefore R is transitive. Hence, R is reflexive, symmetric and transitive Let (x, y), (y, z) \in R \\ \implies x is the wife of y and y is the wife of z.\\ This case is not possible. Also, this does not imply that x is the wife of z.\\ \therefore (x, z) \notin R\\ \therefore R is not transitive.\\ Hence, R is neither reflexive, nor symmetric, nor transitive.\\ (e) R = \{(x, y): x is the father of y\}$$\\$ (x, x) $\notin$ R As x cannot be the father of himself.$\\$ $\therefore$ R is not reflexive.$\\$ Now, let (x, y) $\notin$ R.$\\$ $\implies$ x is the father of y.$\\$ $\implies$ y cannot be the father of y.$\\$ Indeed, y is the son or the daughter of y.$\\$ $\therefore (y, x) \notin$ R $\\$ $\therefore$ R is not symmetric.$\\$ Now, let (x, y) $\in$ R and (y, z) $\notin$ R. $\\$ $\implies$ x is the father of y and y is the father of z.$\\$ $\implies$ x is not the father of z.$\\$ Indeed, x is the grandfather of z.$\\$ $\therefore$ (x, z) $\notin$ R $\\$ $\therefore$ R is not transitive. $\\$ Hence, R is neither reflexive, nor symmetric, nor transitive.

$(iii) A = \{1, 2, 3, 4, 5, 6\}$$\\ R = \{(x, y): y is divisible by x\}$$\\$ We know that any number (x) is divisible by itself.$\\$ So, $(x, x) \in R\\ \therefore R$ is reflexive.$\\$ Now, $(2, 4) \in R$ [as 4 is divisible by 2]$\\$ But, $(4, 2) \notin R$. [as 2 is not divisible by 4]$\\$ $\therefore R$ is not symmetric.$\\$ Let $(x, y), (y, z) \in R$. Then, y is divisible by x and z is divisible by y. $\therefore z$ is divisible by x.$\\$ $\implies (x, z) \in R\\ \therefore R$ is transitive.$\\$ Hence, R is reflexive and transitive but not symmetric.

2   Show that the relation R in the set R of real numbers, defined as R = $\{(a, b): a \leq b ^2 \}$ is neither reflexive nor symmetric nor transitive.

##### Solution :

(i) R = $\{(a, b): a \leq b ^2 \}$$\\ It can be observed that (\dfrac{1}{2},\dfrac{1}{2}) \notin R$$\\$ since ,$\dfrac{1}{2}>(\dfrac{1}{2})^2$\\$R is not reflexive. Now, (1, 4)$\notin $R as 1$<$42 But, 4 is not less than 1 2 .$\\\therefore (4, 1) \notin R$\\$ $\therefore$ R is not symmetric.$\\$ Now, [as 3 < 2 2 = 4 and 2 < (1.5) 2 = 2.25]$\\$ (3, 2), (2, 1.5) $\in$ R$\\$ But, 3 > (1.5) 2 = 2.25$\\$ $\therefore (3, 1.5) \notin$ R$\\$ $\therefore$ R is not transitive.$\\$ Hence, R is neither reflexive, nor symmetric, nor transitive.

3   Check whether the relation R defined in the set $\{1, 2, 3, 4, 5, 6\}$ as R = $\{(a, b): b = a + 1\}$ is reflexive, symmetric or transitive.

Let A =$\{1, 2, 3, 4, 5, 6\}$$\\. A relation R is defined on set A as: R = \{(a, b): b = a + 1\}$$\\$ $\therefore$ R = $\{(1, 2), (2, 3), (3, 4), (4, 5), (5, 6)\}$$\\ we can find (a, a) \notin R, where a \in A.\\ For instance, (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6) \notin R\\ \therefore R is not reflexive.\\ It can be observed that (1, 2) \in R, but (2, 1) \notin R.\\ \therefore R is not symmetric.\\ Now, (1, 2), (2, 3) \in R\\ But, (1, 3) \notin R\\ \therefore R is not transitive\\ Hence, R is neither reflexive, nor symmetric, nor transitive. 4 Show that the relation R in R defined as R = \{(a, b): a \leq b\}, is reflexive and transitive but not symmetric. ##### Solution : R = \{(a, b): a \leq b\}$$\\$ Clearly (a, a) $\in$ R $\\$ [as a = a]$\\$ $\therefore$ R is reflexive.$\\$ Now, (2, 4) $\in$ R (as 2 < 4)$\\$ But, (4, 2) $\notin$ R as $4$ is greater than $2$.$\\$ $\therefore$ R is not symmetric.$\\$ Now, let (a, b), (b, c) $\in$ R.$\\$ Then, a $\leq$ b and b $\leq$ c$\\$ $\implies a \leq c\\ \implies (a, c) \in$ R$\\$ $\therefore$ R is transitive. Hence R is reflexive and transitive but not symmetric

5   Check whether the relation R in R defined as R = $\{(a, b): a \leq b ^3 \}$ is reflexive, symmetric or transitive.

##### Solution :

R = $\{(a, b): a \leq b ^3 \}$$\\ It is observed that(\dfrac{1}{2},\dfrac{1}{2}) \notin R,since, \dfrac{1}{2}>(\dfrac{1}{2})^3$$\\$ $\therefore$ R is not reflexive.$\\$ Now, (1, 2) $\in$ R (as $1 < 2^ 3 = 8$)$\\$ But, (2, 1) $\notin$ R (as $2 ^3 > 1$)$\\$ $\therefore$ R is not symmetric.$\\$ We have $(3,\dfrac{3}{2}),(\dfrac{3}{2},\dfrac{6}{5})\in$R, since $3<(\dfrac{3}{2})^2$ and $\dfrac{3}{2}<(\dfrac{6}{5})^3$$\\ But (3,\dfrac{6}{5}) \notin R as 3> (\dfrac{6}{5})^3$$\\$ $\therefore$ R is not transitive.$\\$ Hence, R is neither reflexive, nor symmetric, nor transitive.

6   Show that the relation R in the set $\{1, 2, 3\}$ given by R = $\{(1, 2), (2, 1)\}$ is symmetric but neither reflexive nor transitive.

##### Solution :

Let A = $\{1, 2, 3\}.$$\\ A relation R on A is defined as R = \{(1, 2), (2, 1)\}$$\\$. It is clear that $(1, 1), (2, 2), (3, 3) \notin$ R.$\\$ $\therefore$ R is not reflexive.$\\$ Now, as $(1, 2) \in$ R and $(2, 1) \in$ R, then R is symmetric.$\\$ Now, $(1, 2)$ and $(2, 1) \in$ R$\\$ However, $(1, 1) \notin$ R$\\$ $\therefore$ R is not transitive.$\\$ Hence, R is symmetric but neither reflexive nor transitive.

7   Show that the relation R in the set A of all the books in a library of a college, given by R = $\{(x, y):$ x and y have same number of pages$\}$ is an equivalence relation.

Set A is the set of all books in the library of a college.$\\$ R = $\{x, y):$ x and y have the same number of pages$\}$$\\ Now, R is reflexive since (x, x) \in R as x and x has the same number of pages.\\ Let (x, y) \in R \implies x and y have the same number of pages.\\ \implies y and x have the same number of pages.\\ \implies (y, x) \in R\\ \therefore R is symmetric.\\ Now, let (x, y) \in R and (y, z) \in R. \implies x and y and have the same number of pages and y and z have the same number of pages.\\ \implies x and z have the same number of pages.\\ \implies (x, z) \in R\\ \therefore R is transitive. Hence, R is an equivalence relation. 8 Show that the relation R in the set A = \{1, 2, 3, 4, 5\} given by R =\{(a, b): |a - b| is even\}, is an equivalence relation. Show that all the elements of \{1, 3, 5\} are related to each other and all the elements of \{2, 4\} are related to each other. But no element of \{1, 3, 5\} is related to any element of \{2, 4\}. ##### Solution : A = \{1, 2, 3, 4, 5\} and R = \{(a, b): |a-b| is even\}$$\\$ It is clear that for any element a $\in$ A, we have |a-a| = 0 (which is even).$\\$ $\therefore$ R is reflexive.$\\$ Let (a, b) $\in$ R.$\\$ $\implies |a - b|$ is even$\\$ $\implies |-(a-b)|= |b - a|$ is also even$\\$ $\implies (b, a) \in$ R$\\$ $therefore$ R is symmetric.$\\$ Now, let (a, b) $\in$ R and (b, c) $\in$ R.$\\$ $\implies$|a-b| is even and |b-c| is even$\\$ $\implies$(a-b) is even and (b-c) is even$\\$ $\implies$(a-c) = (a-b) + (b-c) is even$\\$ [Sum of two even integers is even]$\\$ $\implies |a - b|$ is even.$\\$ $\implies (a, c) \in$ R $\therefore$R is transitive.$\\$ Hence, R is an equivalence relation. Now, all elements of the set $\{1, 2, 3\}$ are related to each other as all the elements of this subset are odd. Thus, the modulus of the difference between any two elements will be even. Similarly, all elements of the set $\{2, 4\}$ are related to each other as all the elements of this subset are even. Also, no element of the subset $\{1, 3, 5\}$ can be related to any element of $\{2, 4\}$ as all elements of $\{1, 3, 5\}$ are odd and all elements of $\{2, 4\}$ are even. Thus, the modulus of the difference between the two elements (from each of these two subsets) will not be even. [as $1 - 2, 1 - 4, 3 - 2, 3 - 4, 5 - 2$ and$5 - 4$ all are odd]

9   Show that each of the relation R in the set A = $\{x \in Z: 0 \leq x \leq 12\}$, given by$\\$ (i) R = $\{(a, b) : |a - b|$ is a multiple of $4\}$$\\ (ii) R = \{(a, b) : a = b\}$$\\$ is an equivalence relation. Find the set of all elements related to $1$ in each case.

##### Solution :

(ii) R = $\{(a, b): a = b\}$$\\ For any element a \in A, we have (a, a) \in R, since a = a. \therefore R is reflexive.\\ Now, let (a, b) \in R.\\ \implies a = b$$\\$ $\implies b = a $$\\$$\implies (b, a) \in$R$\\$ $\therefore$ R is symmetric.$\\$ Now, let (a, b) $\in$ R and (b, c) $\in$ R.$\\$ $\implies a = b$ and $b = c$$\\ \implies a = c\\ \implies (a, c) \in R\\ \therefore R is transitive.\\ Hence, R is an equivalence relation. The elements in R that are related to 1 will be those elements from set A which are equal to 1. Hence, the set of elements related to 1 is \{1\}. Set A = \{x \in Z: 0 \leq x \leq 12\} = \{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12\}$$\\$ (i) R = $\{(a, b): |a - b|$ is a multiple of $4\}$$\\ For any element, a \in A, we have (a, a) \in R as |a -a| \\= 0 is a multiple of 4. \therefore R is reflexive.\\ Now, let (a, b) \in R \implies |a - b| is a multiple of 4.\\ \implies | - (a - b) | = |b - a| is a multiple of 4.\\ \implies (b, a) \in R$$\\$ $\therefore$ R is symmetric. Now, let (a, b), (b, c) $\in$ R.$\\$ $\implies |a - b|$ is a multiple of 4 and |b - c| is a multiple of 4.$\\$ $\implies (a - b)$ is a multiple of 4 and (b - c) is a multiple of 4.$\\$ $\implies (a - c) = (a - b) + (b - c)$ is a multiple of 4.$\\$ $\implies |a - c|$ is a multiple of 4.$\\$ $\implies (a, c) \in$ R$\\$ $\therefore$ R is transitive.$\\$ Hence, R is an equivalence relation.$\\$ The set of elements related to 1 is $\{1, 5, 9\}$ as$\\$ $|1 - 1| = 0$ is a multiple of 4. $|5 - 1| = 4$ is a multiple of 4. $|9 - 1| = 8$ is a multiple of 4.

10   Given an example of a relation. Which is$\\$ (i) Symmetric but neither reflexive nor transitive.$\\$ (ii) Transitive but neither reflexive nor symmetric.$\\$ (iii)Reflexive and symmetric but not transitive.$\\$ (iv) Reflexive and transitive but not symmetric.$\\$ (v) Symmetric and transitive but not reflexive$\\$

##### Solution :

(v) Let A = $\{-5, -6\}$.$\\$ Define a relation R on A as$\\$ R = $\{(-5, -6), (-6, -5), (-5, -5)\}$$\\ Relation R is not reflexive as (-6, -6) \notin R.\\ It is seen that (-5, -6), (-6, -5) \in R. Also, (-5, -5) \in R.\\ \therefore The relation R is transitive.\\ Hence, relation R is symmetric and transitive but not reflexive. (ii) Consider a relation R in R defined as:\\ R = \{(a, b): a < b\}$$\\$ For any a $\in$ R, we have (a, a) $\notin$ R since a cannot be strictly less than a itself.$\\$ In fact, a = a.$\\$ $\therefore$R is not reflexive.$\\$ Now, (1, 2) $\in$ R (as $1 < 2$)$\\$ But, $2$ is not less than $1.$$\\ \therefore (2, 1) \notin R\\ \therefore R is not symmetric.\\ Now, let (a, b), (b, c) \in R.\\ \implies a < b and b < c\\ \implies a < c \\ \implies (a, c) \in R$$\\$ $therefore$R is transitive.$\\$ Hence, relation R is transitive but not reflexive and symmetric.

(iv) Define a relation R in R as:$\\$ R = $\{a, b): a 3 \geq b ^3 \}$$\\ Clearly (a, a) \in R\\ as a^ 3 = a^ 3$$\\$ $\therefore$ R is reflexive.$\\$ Now, (2, 1) $\in$ R [as $2 ^3 \geq 1 ^3$]$\\$ But, (1, 2) $\notin$ R [as $1^ 3 < 2^ 3$ ]$\\$ $\therefore$ R is not symmetric.$\\$ Now, Let (a, b), (b, c) $\in$ R.$\\$ $\implies a^ 3 \leq b^ 3$ and $b ^3 \geq c^ 3$$\\ \implies a ^3 \geq c ^3\\ \implies (a, c) \in R\\ \therefore R is transitive.\\ Hence, relation R is reflexive and transitive but not symmetric. (i) Let A = \{5, 6, 7\}.\\ Define a relation R on A as R = \{(5, 6), (6, 5)\}.\\ Relation R is not reflexive as (5, 5), (6, 6), (7, 7) \notin R. Now, as (5, 6) \in R and also (6, 5) \in R, R is symmetric.\\ \implies (5, 6), (6, 5) \in R, but (5, 5) \notin R\\ \therefore R is not transitive.\\ Hence, relation R is symmetric but not reflexive or transitive. (iii) Let A = \{4, 6, 8\}.\\ Define a relation R on A as\\ A = \{(4, 4), (6, 6), (8, 8), (4, 6), (6, 4), (6, 8), (8, 6)\}$$\\$ Relation R is reflexive since for every a $\in$ A, (a, a) $\in$ R$\\$ i.e., $\{(4, 4), (6, 6), (8, 8)\} \in$ R.$\\$ Relation R is symmetric since (a, b) $\in$ R $\\$ $\implies (b, a) \in$ R for all a, b $\in$ R.$\\$ Relation R is not transitive since $(4, 6), (6, 8) \in$ R, but (4, 8) $\notin$ R. Hence, relation R is reflexive and symmetric but not transitive.

11   Show that the relation R in the set A of points in a plane given by R = $\{(P, Q):$ Distance of the point $P$ from the origin is same as the distance of the point $Q$ from the origin$\}$, is an equivalence relation. Further, show that the set of all point related to a point $P \neq (0, 0)$ is the circle passing through P with origin as centre.

R = $\{(P, Q):$ Distance of point P from the origin is the same as the distance of point $Q$ from the origin$\}$$\\ Clearly, (P, P) \in R since the distance of point P from the origin is always the same as the distance of the same point P from the origin.\\ \therefore R is reflexive.\\ Now, Let (P, Q) \in R.\\ \implies The distance of point P from the origin is the same as the distance of point Q from the origin.\\ \implies The distance of point Q from the origin is the same as the distance of point P from the origin.\\ \implies (Q, P) \in R$$\\$ $\therefore R$ is symmetric.$\\$ Now, Let $(P, Q), (Q, S) \in R.$$\\ \implies The distance of points P and Q from the origin is the same and also, the distance of points Q and S from the origin is the same.\\ \implies The distance of points P and S from the origin is the same.\\ \implies (P, S) \in R\\ \therefore R is transitive.\\ Therefore, R is an equivalence relation.\\ The set of all points related to P \neq (0, 0) will be those points whose distance from the origin is the same as the distance of point P from the origin.\\ In other words, if O (0, 0) is the origin and OP = k, then the set of all points related to P is at a distance of k from the origin.\\ Hence, this set of points forms a circle with the centre as the origin and this circle passes through point P. 12 Show that the relation R defined in the set A of all triangles as R = \{(T _1 , T_ 2 ): T _1 is similar to T_ 2 \}, is equivalence relation. Consider three right angle triangles T_ 1 with sides 3, 4, 5, T_ 2 with sides 5, 12, 13 and T_ 3 with sides 6, 8, 10. Which triangles among T_ 1 , T_ 2 and T_ 3 are related? ##### Solution : R = \{(T _1 , T_ 2 ): T_ 1 is similar to T _2 \}$$\\$ $R$ is reflexive since every triangle is similar to itself.$\\$ Further, If $(T_ 1 , T _2 ) \in R$, then $T_ 1$ is similar to$T_ 2$ $\\$. $\implies T_ 2 4$ is similar to $T_ 1$ .$\\$ $\implies (T_ 2 , T_ 1 ) \in R\\ \therefore R$ is symmetric.$\\$ Now, Let $(T _1 , T _2 ), (T _2 , T_ 3 ) \in R.$$\\ \implies T_ 1 is similar to T_ 2 and T_ 2 is similar to T_ 3 \\. \implies T_ 1 is similar to T_3.$$\\$ $\implies (T_ 1 , T_ 3 ) \in R\\ \therefore R$ is transitive.$\\$ Thus, R is an equivalence relation. Now,$\\$ We can observe that $\dfrac{3}{6}=\dfrac{4}{8}=\dfrac{5}{10}(=\dfrac{1}{2})\\ \therefore$The corresponding sides of triangles $T _1$ and $T_ 3$ are in the same ratio.$\\$ Then, triangle $T _1$ is similar to triangle $T_ 3$ $\\$. Hence, $T_ 1$ is related to $T _3$ $\\$.

13   Show that the relation $R$ defined in the set $A$ of all polygons as $R = \{(P _1 , P _2 ): P_ 1$ and $P_ 2$ have same number of sides$\}$, is an equivalence relation. What is the set of all elements in $A$ related to the right angle triangle $T$ with sides $3, 4$ and $5$?

##### Solution :

$(v) f: Z \to Z$ is given by $f(x) = x ^3$$\\ It is seen that for x, y \in Z, f(x) = f(y) \\ x ^3 = y ^3 \\ x = y.\\ \therefore f is injective.\\ Now, 2 \in Z. But, there does not exist any element x \in Z such that\\ f(x) = 2\\ or \ x ^3 = 2.\\ \therefore f is not surjective.\\ Hence, function f is injective but not surjective. (ii) f: Z \to Z is given by f(x) = x^ 2$$\\$ It is seen that $f(-1) = f(1) = 1$, but $-1 \neq 1.$$\\ \therefore f is not injective.\\ Now, -2 \in Z. But, there does not exist any element x \in Z such that \\ f(x) = -2 or x 2 = -2.$$\\$ $\therefore f$ is not surjective. Hence, function $f$ is neither injective nor surjective.

21   Show that the Signum Function $f: R \to R$, given by f(x) =$\begin{cases} 1 & \quad \text{if }X>0\\ 0 & \quad \text{if } X=0\\ -1 &\quad \text{if} X<0 \end{cases}$$\\ is neither one-one nor onto. ##### Solution : f: R \to R is given by f(x) =\begin{cases} 1 & \quad \text{if }X>0\\ 0 & \quad \text{if } X=0\\ -1 &\quad \text{if} X<0 \end{cases}$$\\$ It is seen that f(1) = f(2) = 1, but 1 $\neq$ 2.$\\$ $\therefore f$ is not one - one.$\\$ Now, as f(x) takes only 3 values (1, 0, or -1) for the element -2 in co-domain $R$, there does not exist any x in domain R such that f(x) = -2.$\\$ $\therefore f$ is not onto.$\\$ Hence, the Signum function is neither one - one nor onto

22   Let $A = \{1, 2, 3\}, B = \{4, 5, 6, 7\}$ and let $f = \{(1, 4), (2, 5), (3, 6)\}$ be a function from $A$ to $B$. Show that f is one - one.

##### Solution :

It is given that $A = \{1, 2, 3\}, B = \{4, 5, 6, 7\}.$$\\ f: A\to B is defined as f = \{(1, 4), (2, 5), (3, 6)\}.\\ \therefore f (1) = 4, f (2) = 5, f (3) = 6$$\\$ It is seen that the images of distinct elements of $A$ under $f$ are distinct. Hence, function f is one -one.

23   In each of the following cases, state whether the function is one - one, onto or bijective. Justify your answer.$\\$ $(i) f: R \to R$ defined by f(x) = 3 - 4x$\\$ $(ii) f: R \to R$ defined by f(x) = 1 + x$^2$

25   Let $f: N \to N$ be defined by $f(n)=\begin{cases} \dfrac{n+1}{2} & \quad \text{if } n \text{ is odd}\\ \ \ \ \ \ \ \ \ \text{for all n$\in$N} \ \ \ \ \ \ \ \ \ \\\ \dfrac{n}{2} & \quad \text{if } n \text{ is even} \end{cases}$$\\ State whether the function f is bijective. Justify your answer. ##### Solution : f: N \to N be defined by f(n)=\begin{cases} \dfrac{n+1}{2} & \quad \text{if } n \text{ is odd}\\ \ \ \ \ \ \ \ \ \text{for all n \in N} \ \ \ \ \ \ \ \ \ \\\ \dfrac{n}{2} & \quad \text{if } n \text{ is even} \end{cases}$$\\$It can be observed that:$\\$ $f(1) =\dfrac{1+1}{2} =1$ and $f(2)=\dfrac{2}{2} =1$[By definition of f(n)]$\\$ $f(1) = f(2),$ where $1 \neq 2$$\\ \therefore f is not one-one.\\ Consider a natural number (n) in co-domain N.\\ Case I: n is odd\\ \therefore n = 2r + 1 for some r \in N. Then, there exists 4r + 1\in N such that\\ f(4r+1) =\dfrac{4r+1+1}{2}=2r+1$$\\$ Case II: n is even$\\$ $\therefore n = 2r$ for some $r \in N$. Then, there exists $4r \in N$ such that$\\$ $f(4r) =\dfrac{4r}{2}=2r\\ \therefore f$ is onto.$\\$Hence, f is not a bijective function.

26   Let $A = R - \{3\}$ and $B = R - \{1\}$. Consider the function $f: A \to B$ defined by (x)=$(\dfrac{x-2}{x-3})$Is f one-one and onto? Justify your answer.

$A = R - \{3\}, B = R - \{1\}$ and $f: A \to B$ defined by f(x) =$(\dfrac{x-2}{x-3})$$\\ Let x, y \in A such that f(x) = f(y)\\ \dfrac{X-2}{X-3}=\dfrac{Y-2}{Y-3}\\(x - 2)(y - 3) = (y - 2)(x - 3) \\ xy - 3x - 2y + 6 = xy - 2x - 3y + 6\\ - 3x - 2y = - 2x - 3y \\ x = y\\ \therefore f is one-one.\\ Let y \in B = R - \{1\}. Then, y \neq 1.\\ The function f is onto if there exists x \in A such that f(x) = y. Now, f(x) = y \\ \dfrac{X-2}{X-3} =y\\ x - 2 = xy - 3y \\ x(1 - y) = - 3y + 2\\x=\dfrac{2-3y}{1-y}\in A \ \ \ \ \ \ [y\neq 1]$$\\$ Thus, for any y $\in$ B, there exists $\dfrac{2-3y}{1-y}\in A$ such that $\\$ $f \dfrac{2-3y}{1-y}=\dfrac{\dfrac{2-3y}{1-y}-2}{\dfrac{2-3y}{1-y}-3} \\ =\dfrac{2-3y-2+2y}{2-3y-3+3y} =\dfrac{-y}{-1}=y$$\\ \therefore f is onto.\\ Hence, function f is one - one and onto. 27 Let f: R\to R be defined as f(x) = x_ 4 . Choose the correct answer.\\ (A) f is one-one onto\\ (B) f is many-one onto\\ (C) f is one-one but not onto\\ (D) f is neither one-one nor onto ##### Solution : f: R \to R is defined as f(x) = x _4 .\\ Let x, y \in R such that f(x) = f(y).\\ x_ 4 = y_ 4\\ x = \pm y\\ \therefore f(x) = f(y) does not imply that x = y.\\ For example f(1) = f(-1) = 1 \therefore f is not one-one.\\ Consider an element 2 in co-domain R. It is clear that there does not exist any x in domain R\\ such that f(x) = 2.$$\\$ $\therefore f$ is not onto.$\\$ Hence, function f is neither one - one nor onto. The correct answer is D.

28   Let $f: R \to R$ be defined as $f(x) = 3x$. Choose the correct answer.$\\$ (A) $f$ is one -one onto$\\$ (B) $f$ is many - one onto$\\$ (C) $f$ is one -one but not onto $\\$(D) $f$ is neither one - one nor onto

##### Solution :

$f: R \to R$ is defined as $f(x) = 3x.$$\\ Let x, y \in R such that f(x) = f(y).$$\\$ $3x = 3y\\ x = y\\ \therefore f$ is one-one.$\\$ Also, for any real number (y) in co-domain $R$, there exists $y3$ in $R$ such that$\\$ $\therefore f$ is onto.$\\$ Hence, function f is one - one and onto.$\\$ The correct answer is A.

29   Let $f: \{1, 3, 4\} \to \{1, 2, 5\}$ and $g: \{1, 2, 5\} \to \{1, 3\}$ be given by $f = \{(1, 2), (3, 5), (4, 1)\}$ and $g = \{(1, 3), (2, 3), (5, 1)\}$. Write down gof.

##### Solution :

The functions $f: \{1, 3, 4\} \to \{1, 2, 5\}$ and $g: \{1, 2, 5\} \to \{1, 3\}$ are defined as$\\$ $f = \{(1, 2), (3, 5), (4, 1)\}$ and $g = \{(1, 3), (2, 3), (5, 1)\}$.$\\$ gof $(1) = g[f(1)] = g(2) = 3 \ \ \ \ \ \ \ \$ [as $f(1) = 2$ and $g(2) = 3$]$\\$ gof $(3) = g[f(3)] = g(5) = 1 \ \ \ \ \ \ \$ [as $f(3) = 5$ and $g(5) = 1$]$\\$ gof $(4) = g[f(4)] = g(1) = 3 \ \ \ \ \ \ \ \ \$ [as $f(4) = 1$ and $g(1) = 3$]$\\$ $\therefore$ gof = $\{(1, 3), (3, 1), (4, 3)\}$

31   Find $gof$ and $fog$, if$\\$ $(i) (x) = |x|$ and $(x) = |5x - 2|$$\\ (ii) (x) = 8x^3 and (x) = x ^\frac{1}{3} ##### Solution : (i) f(x) = |x| and g(x) = |5x-2|$$\\$ $\therefore go f(x) = g(f(x)) = g(|x|) = |5|x|-2|\\ fog(x) = f(g(x)) = f(|5x-2|) = ||5x-2|| = |5x-2|$$\\ (ii) f(x) = 8x^3 and g(x) = x ^\frac{1}{3}\\ \therefore gof(x) = g(f(x)) = g(8x ^3 ) = (8X^3 )^\frac{1}{ 3} = 2x\\ fog(x) = f(g(x)) = f( x ^\frac{1}{3} ) ^3 = 8( x^\frac{1}{ 3} ) ^3 = 8x 32 If (x) = (4x+3) (6x-4), x \neq \dfrac{2}{3},show that fof(x) = x, for all x \neq \dfrac{2}{3}.What is the inverse of f? ##### Solution : It is given that f(x) =\dfrac{4x+3}{6x-4} ,x \neq \dfrac{2}{3}\\= (fof )(x) =f(f(x)) =f(\dfrac{4x+3}{6x-4})= \\ \dfrac{4(\dfrac{4x+3}{6x-4})+3}{6(dfrac{4x+3}{6x-4})-4}\\= \dfrac{16x+12+18x-12}{24x+18-24s+16} =\dfrac{34x}{34}\\ \therefore fof(x) = x, for all x \neq \dfrac{2}{3}\\ fof = I _x Hence, the given function f is invertible and the inverse of f is f itself 33 State with reason whether following functions have inverse\\ (i) f: \{1, 2, 3, 4\} \to \{10\} with \\ f = \{(1, 10), (2, 10), (3, 10), (4, 10)\} \\ (ii) g: \{5, 6, 7, 8\} \to \{1, 2, 3, 4\} with \\ g = \{(5, 4), (6, 3), (7, 4), (8, 2)\} \\ (iii) h: \{2, 3, 4, 5\} \to \{7, 9, 11, 13\} with\\ h = \{(2, 7), (3, 9), (4, 11), (5, 13)\} ##### Solution : (i) f: \{1, 2, 3, 4\} \to \{10\} defined as f = \{(1, 10), (2, 10), (3, 10), (4, 10)\}$$\\$ From the given definition of $f$, we can see that $f$ is a many one function as$\\$ $f(1) = f(2) = f(3) = f(4) = 10\\ \therefore f$ is not one - one.$\\$ Hence, function $f$ does not have an inverse.$\\$ $\\$ $(ii) g: \{5, 6, 7, 8\} \to \{1, 2, 3, 4\}$ defined as$\\$ $g = \{(5, 4), (6, 3), (7, 4), (8, 2)\}$$\\ From the given definition of g, it is seen that g is a many one function as g(5) = g(7) = 4.$$\\$ $\therefore g$ is not one - one.$\\$ Hence, function g does not have an inverse.$\\$ $\\$ $(iii) h: \{2, 3, 4, 5\} \to \{7, 9, 11, 13\}$ defined as$\\$ $h = \{(2, 7), (3, 9), (4, 11), (5, 13)\}$$\\ It is seen that all distinct elements of the set \{2, 3, 4, 5\} have distinct images under h.\\ \therefore Function h is one - one.\\ Also, h is onto since for every element y of the set \{7, 9, 11, 13\}, there exists an element x in the set \{2, 3, 4, 5\}, such that h(x) = y.$$\\$ Thus, $h$ is a one - one and onto function.$\\$ Hence, $h$ has an inverse

34   Show that $f: [-1, 1] \to R$, given by$(x) = X(X+2)$ is one - one. Find the inverse of the function $f: [-1, 1] \to$ Range $f$.$\\$ (Hint: For y $\in$ Range f, y = (x) = (X+2)) , for some x in [-1, 1], i.e., x = 2y(1-y))

$f: [-1, 1] \to R$ is given as $(x) = X(X+2)$$\\ For one - one\\ Let f(x) = f(y)\\ (X+2) = (Y+2)\\ xy +2x = xy +2y\\ 2x = 2y\\ x = y\\ \therefore f is a one - one function.\\ It is clear that f: [-1, 1] \to Range f is onto.\\ \therefore f: [-1, 1] \to Range f is one - one and onto and therefore, the inverse of the function f: [-1, 1] \to Range f exists.\\ Let g: Range f \to [-1, 1] be the inverse of f.\\ Let y be an arbitrary element of range f.\\ Since f: [-1, 1] \to Range f is onto, we have\\ y = f(x) for some x \in [-1, 1]$$\\$ $y=\dfrac{x}{x+2}\\ xy + 2 y = x\\ x(1-y)=2y\\ x-\dfrac{2y}{1-y},y\neq 1$$\\ Now, let us define g: Range f \to [-1, 1] as g ( y ) =\dfrac{2y}{1-y},y \neq 1 Now ,( gof ) ( x ) = g ( f ( x ))\\ =g(\dfrac{x}{(x+2)}) \\ =2(\dfrac{2(dfrac{x}{x+2})}{1-(\dfrac{x}{x+2})})\\ =\dfrac{2x}{x+2-x} \\ =x\\ (fog ) ( y ) = f ( g ( y )) =f(\dfrac{2y}{1-y})\\ =\dfrac{\dfrac{2y}{1-y}}{\dfrac{2y}{1-y}+2} \\ =\dfrac{2y}{2y+2-2y} =y\\ \therefore gof=x=I_[-1,1] and fog =y=I_\text{Range f}\\ \therefore f^-1 =g\\ f^-1(y) =\dfrac{2y}{1-y},y\neq 1 35 Consider f: R \to R given by f(x) = 4x + 3. Show that f is invertible. Find the inverse of f. ##### Solution : f: R \to R is given by, f(x) = 4x + 3$$\\$ For one - one$\\$ Let $f(x) = f(y)\\ 4x + 3 = 4y + 3\\ 4x = 4y\\ x = y\\ \therefore f$is a one - one function$\\$ For onto For $y \in R$, let $y = 4x + 3.\\ x=\dfrac{y-3}{4}\in R$$\\ Therefore, for any y \in R, there exists x=\dfrac{y-3}{4}\in R,such that \\ f(x)=f(\dfrac{y-3}{4})=4(\dfrac{y-3}{4}) +3=y\\ \therefore f is onto.\\ Thus, f is one - one and onto and therefore, f -1 exists.\\ Let us define g: R \to R by g(x) = \dfrac{y-3}{4}$$\\$ Now , $\\$ $(gof)(x) = g(f(x)) = g(4x + 3)=\dfrac{(4x+3)-3}{4}=\dfrac{4x}{4}=x$$\\ and (fog)(y) = f(g(y)) =f(\dfrac{y-3}{4})=4(\dfrac{y-3}{4})+3=y-3+3=y\\ \therefore gof = fog= I_r$$\\$ Hence, $f$ is invertible and the inverse of $f$ is given by $f -1 (y) = g(y) =\dfrac{y-3}{4}$

36   Consider $f: R+ \to [4, \infty)$ given by $f(x) = x^ 2 + 4$. Show that f is invertible with the inverse $f-1$ of given $f$ by$f -1 (y) = \sqrt{Y - 4}$ , where $R+$ is the set of all non-negative real numbers.

$f: R + \to [4, \infty)$ is given as $f(x) = x ^2 + 4.$$\\ For one - one\\ Let f(x) = f(y)\\ x^ 2 + 4 = y ^2 + 4\\ x^ 2 = y^ 2\\ x = y [as x = y \in R + ]\\ \therefore f is a one - one function.\\ For onto\\ For y \in [4, \infty), let y = x^ 2 + 4\\ x^ 2 = y - 4 \geq 0 [as y \geq 4]\\ x = \sqrt{Y - 4} \geq 0$$\\$ Therefore, for any $y \in [4, \infty )$, there exists $x = \sqrt{Y -4} \in R +$ , such that$\\$ $f(x) = f( \sqrt{Y -4} ) = ( \sqrt{Y - 4} )^ 2 + 4 = y - 4 + 4 = y\\ \therefore f$ is onto.Thus, $f$ is one - one and onto and therefore, $f -1$ exists.$\\$ Let us define $g: [4, \infty) \to R+$ by $g(y) = \sqrt{Y - 4}$$\\ Now,\\ (gof)(x) = g(f(x)) = g(x^ 2 + 4) = \sqrt{}(x 2 +4)-4 = \sqrt{} \sqrt{X ^2} =x$$\\$ and$\\$ $(fog)(y) = f(g(y)) = f(\sqrt{ Y - 4} ) = ( \sqrt{Y -4 ) ^2 }+ 4 = y - 4 + 4 = y\\ \therefore gof = fog = I_ R$$\\ Hence, f is invertible and the inverse of f is given by f -1 (y) = g(y) = \sqrt{Y - 4} 37 Consider f: R + \to [-5, \infty ) given by f(x) = 9x ^2 + 6x - 5. Show that f is invertible with f-1 (y)=\left(\dfrac{(\sqrt{y+6})-1}{3}\right) ##### Solution : f: R + \to [-5, \infty) is given as f(x) = 9x^ 2 + 6x - 5.$$\\$ Let $y$ be an arbitrary element of $[-5, \infty).$$\\ Let y = 9x ^2 + 6x - 5\\ y = (3x + 1)^ 2 -1-5 = (3x + 1)^ 2 -6\\ y + 6 = (3x + 1) ^2\\ 3x + 1 =\sqrt{Y+6}$$\\$ [as $y \geq -5 \implies y + 6 > 0]\\ x=\left(\dfrac{(\sqrt{y+6})-1}{3}\right)\\ \therefore f$ is onto, thereby range $f = [-5, \infty ).$$\\ Let us define g: [-5, \infty ) \to R+ as \\$$g(y)=\left(\dfrac{(\sqrt{y+6})-1}{3}\right)$$\\ Now, (gof)(x) = g(f(x)) = g(9x^ 2 + 6x-5) = g((3x + 1) ^2 -6) =\sqrt{(3x+1)^2-6+6-1}\\ =\dfrac{3x+1-1}{3} =\dfrac{3x}{3}=X$$\\$ and $\\$$(fog)(y) = f(g(y))=\left(\dfrac{\sqrt{Y+6-1}}{3}\right)\\ =\left[3(\dfrac{\sqrt{Y+6}-1}{3})+1^2\right]-6\\ =(\sqrt{Y+6})^2-6=+6-6=y\\ \therefore gof = x = I_ R and fog = y = l_\text{ Range f}$$\\$ Hence, $f$ is invertible and the inverse of $f$ is given by $f^-1(y) =g(y)=(\dfrac{\sqrt{Y+6}-1}{3})$

38   Let $f: X \to Y$ be an invertible function. Show that f has unique inverse. (Hint: suppose $g_ 1$ and $g_ 2$ are two inverses of $f$. Then for all $y \in Y, fog _1 (y) = I_ Y (y) = fog 2 (y).$ Use one -one ness of $f$ ).

##### Solution :

Let $f: X \to Y$ be an invertible function.$\\$ Also, suppose f has two inverses (say $g _1$ and $g_ 2$ )$\\$ Then, for all $y \in Y$, we have$\\$ $fog1(y) = I_ Y (y) = fog2(y)\\ f(g _1 (y)) = f(g _2 (y))\\ g_ 1 (y) = g _2 (y)$ [as $f$ is invertible $\implies f$ is one - one]$\\$ $g _1 = g _2$ [as $g$ is one - one]$\\$ Hence, $f$ has a unique inverse.

39   Consider $f: \{1, 2, 3\} \to \{a, b, c\}$ given by $f(1) = a, f(2) = b$ and $f(3) = c$. Find $f -1$ and show that $(f -1 ) -1 = f$.

##### Solution :

Function $f: \{1, 2, 3\} \to \{a, b, c\}$ is given by $f(1) = a, f(2) = b$, and $f(3) = c$$\\ If we define g: \{a, b, c\} \to \{1, 2, 3\} as g(a) = 1, g(b) = 2, g(c) = 3.$$\\$ We have$\\$ $(fog)(a) = f(g(a)) = f(1) = a\\ (fog)(b) = f(g(b)) = f(2) = b\\ (fog)(c) = f(g(c)) = f(3) = c$$\\ and\\ (gof)(1) = g(f(1)) = f(a) = 1\\ (gof)(2) = g(f(2)) = f(b) = 2\\ (gof)(3) = g(f(3)) = f(c) = 3\\ \therefore gof = I_ X and fog = I_ Y , where X = \{1, 2, 3\} and Y= \{a, b, c\}.\\ Thus, the inverse of f exists and f-1 = g.$$\\$ $\therefore f -1 :\{ a, b, c\} \to \{1, 2, 3\}$ is given by $f ^-1 (a) = 1, f ^-1 (b) = 2, f ^-1 (c) = 3$$\\ Let us now find the inverse of f -1 i.e., find the inverse of g. If we define h: \{1, 2, 3\} \to \{a, b, c\} as h (1) = a, h (2) = b, h (3) = c$$\\$ We have $(goh)(1) = g(h(1)) = g(a) = 1\\ (goh)(2) = g(h(2)) = g(b) = 2\\ (goh)(3) = g(h(3)) = g(c) = 3$$\\ and\\ (hog)(a) = h(g(a)) = h(1) = a\\ (hog)(b) = h(g(b)) = h(2) = b\\ (hog)(c) = h(g(c)) = h(3) = c\\ \therefore goh = I_ X and hog = I_ Y , where X = \{1, 2, 3\} and Y = \{a, b, c\}.$$\\$ Thus, the inverse of $g$ exists and $g -1 = h \implies (f -1 ) -1 = h.$$\\ It can be noted that h = f.$$\\$ Hence, $(f -1 ) -1 = f$

40   Let $f: X \to Y$ be an invertible function. Show that the inverse of $f -1$ is $f$,i.e., $(f -1 ) -1 = f$.

##### Solution :

Let $f: X \to Y$ be an invertible function.$\\$ Then, there exists a function $g: Y \to X$ such that $gof = I_ X$ and $fog = I_ Y .$$\\ Here, f -1 = g.$$\\$ Now,$\\$ $gof = I_ X$ and $fog = I_ Y$$\\ \implies f -1of = I_ X and fof-1 = I_ Y$$\\$ Hence, $f -1 : Y \to X$ is invertible and f is the inverse of $f -1$i.e., $(f -1 ) -1 = f$

41   If $f: R \to R$ be given by $(x) = (3-x 3 )^\dfrac{1}{ 3}$ , then fof (x) is

$f: R \to R$ be given as $f(x) =(3-x^3)^\frac{1}{3}\\ \therefore fof(x) = f(f(x)) = f(3-x ^3 ) \frac{1}{3} = [3-((3-x ^3 ) ^\frac{1}{3} )\frac{1}{ 3} ]\frac{1}{ 3}\\ = [3-(3-x ^3 )] \frac{1}{3} =(x ^3 )\frac{1}{ 3}\\ \therefore fof(x) = x$$\\ The correct answer is C. 42 Let f: R-\{-\frac{4}{3}\}\to R be a function as f(x) =\dfrac{4x}{3x+4}The inverse of f is map g:Range f \to R- \{-\frac{4}{3}\}given by ##### Solution : It is given that f:R -\{-\frac{4}{3}\}\to R be a function as f(x) = \dfrac{4x}{3x+4} Let y be an arbitrary element of Range f.\\ Then, there exists x \in R- \{-\frac{4}{3}\} such that y=F(x)$$\\$ $y=\dfrac{4x}{3x+4}\\ 3xy + 4y=4x\\ x(4-3y)=4y\\ x=\dfrac{4y}{4-3y}$ Let us define g: Range $f \to R- \{-\frac{4}{3}\}$ as $g(y) =\dfrac{4y}{4-3y}$$\\ Now,\\ gof(x) = g(f(x)) = g(\dfrac{4x}{3x+4})\\ =\dfrac{4(\dfrac{4x}{3x+4})}{4-3(\dfrac{4x}{3x+4})}=\\ \dfrac{16x}{12 x + 16 - 12 x}=\dfrac{16x}{16}=x$$\\$ and $fo(y) = f(g(y)) = f(\dfrac{4y}{4-3y}) =\\ dfrac{4(\dfrac{4y}{4-3y})}{3(\dfrac{4y}{4-3y})+4}\\ =\dfrac{16y}{12y+16-12x}=y$$\\ \therefore gof =I_R-\{\dfrac{4}{3}\} and fog =I_\text{Range f}$$\\$ Thus, g is the inverse of f i.e., $f - 1 = g$.$\\$ Hence, the inverse of $f$ is the map g: Range $f \to R-\{-\dfrac{4}{3}\}$ , which is given by $g(y) =\dfrac{4y}{4-3y}$ . The correct answer is $B$.

43   Determine whether or not each of the definition of given below gives a binary operation.$\\$ In the event that * is not a binary operation, give justification for this.$\\$ (i) On Z + , define * by a * b = a - b$\\$ (ii) On Z + , define * by a * b = ab$\\$ (iii) On R, define * by a * b = ab 2$\\$ (iv) On Z + , define * by a * b = |a - b|$\\$ v) On Z + , define * by a * b = a

(i) On Z + , * is defined by a * b = a - b.$\\$ It is not a binary operation as the image of (1, 2) under * is 1 * 2 = 1 - 2 = -1$\notin$ Z + .$\\$ $\\$ (ii) On Z + , * is defined by a * b = ab.$\\$ It is seen that for each a, b $\in$ Z + , there is a unique element ab in Z + .$\\$ This means that * carries each pair (a, b) to a unique element a * b = ab in Z + .$\\$ Therefore, * is a binary operation.$\\$$\\ (iii) On R, * is defined by a * b = ab 2 .\\ It is seen that for each a, b \in R, there is a unique element ab 2 in R.\\ This means that * carries each pair (a, b) to a unique element a * b = ab 2 in R.\\ Therefore, * is a binary operation.\\$$\\$ (iv) On Z + , * is defined by a * b = |a - b|.$\\$ It is seen that for each a, b $\in$ Z+, there is a unique element |a - b| in Z + .$\\$ This means that * carries each pair (a, b) to a unique element a * b = |a - b| in Z+. Therefore, * is a binary operation.$\\$$\\ (v) On Z + , * is defined by a * b = a.\\ It is seen that for each a, b \in Z + , there is a unique element a in Z + .\\ This means that * carries each pair (a, b) to a unique element a * b = a in Z + .\\ Therefore, * is a binary operation. 44 For each binary operation * defined below, determine whether * is commutative or associative.\\ (i) On Z, define a * b = a - b\\ (ii) On Q , define a * b = ab + 1\\ (iii) On Q , define a * b = ab2\\ (iv) On Z +, define a * b = 2ab\\ (v) On Z+, define a * b = ab\\ (vi) On R - {-1}, define a * b = ab+1 ##### Solution : (iv)On Z + , * is defined by a * b = 2ab.\\ It is known that: ab = ba for all a, b \in Z+\\ 2ab = 2ba for all a, b \in Z+\\ a * b = b * a for all a, b \in Z+\\ Therefore, the operation * is commutative.\\ It can be observed that\\ (1 * 2) * 3 = 2 ^{1×2 }* 3 = 4 * 3 = 2^{ 4×3} = 2^ 12 and 1 * (2 * 3) = 1 * 22×3 = 1 * 26 = 1 * 64 = 21×64 = 264\\ \therefore (1 * 2) * 3 \neq 1 * (2 *3), where 1, 2, 3 \in Z +\\ Therefore, the operation * is not associative.\\$$\\$ (v) On Z + , * is defined by a * b = a b .$\\$ It can be observed that$\\$ $1*2 = 1^ 2 = 1$ and $2*1 = 2^ 1 = 2\\ \therefore 1 * 2 \neq 2 * 1$, where 1, 2 $\in$ Z+$\\$ Therefore, the operation * is not commutative.$\\$ It can also be observed that$\\$ $(2 *3) * 4 = 2 ^{3 * 4} = 8 * 4 = 8 ^4 = 2^ 12$ and $2 * (3 * 4) = 2 * 3 ^4 = 2 * 81 = 2^ 81\\ \therefore (2 * 3) * 4 \neq 2 * (3 * 4)$, where 2, 3, 4 $\in$ Z +$\\$ Therefore, the operation * is not associative.$\\$ (vi) On R, * - $\{-1\}$ is defined by a * b = ab+1$\\$ It can be observed that$\\$ $1 * 2 = 12+1=13$ and $2 * 1 = 21+1=22=1\\ \therefore 1 * 2 \neq 2 * 1$, where 1, 2 $\in$ R -$\{-1\}$ Therefore, the operation * is not commutative.$\\$ It can also be observed that$\\$ $(1*2)*3=\dfrac{1}{2+1}*3=\dfrac{1}{3}*3\\ =\dfrac{\dfrac{1}{3}}{3+1}=\dfrac{1}{12}$$\\ and \\ 1*(2*3) =1*\frac{2}{3+1}=1*\frac{2}{4}=1*\frac{1}{2}=\\ \dfrac{1}{\frac{1}{2}+1}\\ =\dfrac{1}{\frac{3}{2}} =\frac{2}{3}$$\\$ $\therefore (1*2)*3\neq 1*(2*3)$,where 1, 2, 3 $\in$ R - $\{-1\}$$\\ Therefore, the operation * is not associative. (i) On Z, * is defined by a * b = a - b.\\ It can be observed that 1 * 2 = 1 - 2 = -1 and 2 * 1 = 2 - 1 = 1.\\ \therefore 1 * 2 \neq 2 * 1, where 1, 2 \in Z\\ Hence, the operation * is not commutative.\\ Also, we have\\ (1 * 2) * 3 = (1 - 2) * 3 = -1 * 3 = -1 - 3 = -4\\ 1 * (2 * 3) = 1 * (2 - 3) = 1 * -1 = 1 - (-1) = 2\\ \therefore (1 * 2) * 3 \neq 1 * (2 * 3), where 1, 2, 3 \in Z\\ Hence, the operation * is not associative.\\ (ii) On Q, * is defined by a * b = ab + 1.\\ It is known that: ab = ba for all a, b \in Q\\ ab + 1 = ba + 1 for all a, b \in Q\\ a * b = a * b for all a, b \in Q\\ Therefore, the operation * is commutative.\\ It can be observed that\\ (1 * 2) * 3 = (1 × 2 + 1) * 3 = 3 * 3 = 3 × 3 + 1 = 10\\ 1 * (2 * 3) = 1 * (2 × 3 + 1) = 1 * 7 = 1 × 7 + 1 = 8\\ \therefore (1 * 2) * 3 \neq 1 * (2 * 3), where 1, 2, 3 \in Q\\ Therefore, the operation * is not associative.\\ (iii) On Q, * is defined by a * b = \frac{ab} {2}$$\\$ It is known that: ab = ba for all a, b $\in$ Q$\\$ $\dfrac{ab}{2}=\dfrac{ab}{2}$ for all a,b $\in$Q$\\$ a * b = b * a for all a, b $\in$ Q$\\$ Therefore, the operation * is commutative.$\\$ For all a, b, c $\in$ Q, we have $\\$ $(a*b)*c=(\frac{ab}{2})*c=\dfrac{(\frac{ab}{2})c}{2}=\dfrac{abc}{4}$$\\ a*(b*c)=a*(\dfrac{bc}{2})=\dfrac{a(\dfrac{bc}{2})}{2}=\dfrac{abc}{4}$$\\$ $\therefore (a*b)*c = a*(b*c)$, where a, b, c $\in$ Q Therefore, the operation * is associative.

45   Consider the binary operation $\Lambda$ on the set $\{1, 2, 3, 4, 5\}$ defined by a $\Lambda$ b = min {a, b}. Write the operation table of the operation $\Lambda$.

##### Solution :

The binary operation $\Lambda$ on the set $\{1, 2, 3, 4, 5\}$ is defined as a $\Lambda$ b = min $\{a, b\}$ for all a, b $\in \{1, 2, 3, 4, 5\}$.$\\$ Thus, the operation table for the given operation $\Lambda$ can be given as:$\\$

46   Consider a binary operation * on the set $\{1, 2, 3, 4, 5\}$ given by the following multiplication table.$\\$ (i) Compute (2 * 3) * 4 and 2 * (3 * 4)$\\$ (ii) Is * commutative?$\\$ (iii) Compute (2 * 3) * (4 * 5).$\\$ (Hint: use the following table)

$(i) (2 * 3) * 4 = 1 * 4 = 1\\ 2 * (3 * 4) = 2 * 1 = 1$$\\ (ii) For every a, b \in \{1, 2, 3, 4, 5\}, we have a * b = b * a. Therefore, the operation * is commutative.\\ (iii) (2 * 3) = 1 and (4 * 5) = 1\\ \therefore (2 * 3) * (4 * 5) = 1 * 1 = 1 47 Let*\prime be the binary operation on the set \{1, 2, 3, 4, 5\} defined by a ^{*\prime} b = H.C.F. of a and b. Is the operation *\prime same as the operation * defined in Exercise 4 above? Justify your answer. ##### Solution : We observe that the operation tables for the operations * and *\prime are the same. Thus, the operation *\prime is same as the operation*. The binary operation * \prime on the set \{1, 2, 3 4, 5\} is defined as a ^{* \prime } b = H.C.F of a and b. The operation table for the operation * \prime can be given as: 48 Let * be the binary operation on N given by a * b = L.C.M. of a and b. Find\\ (i) 5 * 7, 20 * 16\\ (ii) Is * commutative?\\ (iii) Is * associative?\\ (iv) Find the identity of * in N\\ (v) Which elements of N are invertible for the operation *? ##### Solution : The binary operation * on N is defined as a * b = L.C.M. of a and b. (i) 5 * 7 = L.C.M. of 5 and 7 = 35\\ 20 * 16 = L.C.M of 20 and 16 = 80\\$$\\$ (ii) It is known that$\\$ L.C.M of a and b = L.C.M of b and a for all a, b $\in$ N.$\\$ $\therefore$ a * b = b * a$\\$ Thus, the operation * is commutative.$\\$$\\ (iii) For a, b, c \in N, we have\\ (a * b) * c = (L.C.M of a and b) * c = LCM of a, b, and c\\ a * (b * c) = a * (LCM of b and c) = L.C.M of a, b, and c\\ \therefore (a * b) * c = a * (b * c)\\ Thus, the operation * is associative.\\$$\\$ (iv) It is known that:$\\$ L.C.M. of a and 1 = a = L.C.M. 1 and a for all a $\in$ N$\\$ a * 1 = a = 1 * a for all a $\in$ N$\\$ Thus, 1 is the identity of * in N.$\\$$\\ (v) An element a in N is invertible with respect to the operation * if there exists an element b in\\ N, such that a * b = e = b * a.\\ Here, e = 1\\ This means that L.C.M of a and b = 1 = L.C.M of b and a\\ This case is possible only when a and b are equal to 1.\\ Thus, 1 is the only invertible element of N with respect to the operation *. 49 A = N × N and * be the binary operation on A defined by (a, b) * (c, d) = (a + c, b + d) Show that * is commutative and associative. Find the identity element for * on A, if any. ##### Solution : A = N × N and * is a binary operation on A and is defined by\\ (a, b) * (c, d) = (a + c, b + d)\\ Let (a, b), (c, d) \in A\\ Then, a, b, c, d \in N\\ We have:\\ (a, b) * (c, d) = (a + c, b + d)\\ (c, d) * (a, b) = (c + a, d + b) = (a + c, b + d)\\ [Addition is commutative in the set of natural numbers] \therefore (a, b) * (c, d) = (c, d) * (a, b)$$\\$ Therefore, the operation * is commutative.$\\$ Now, let (a, b), (c, d), (e, f) $\in$ A$\\$ Then, a, b, c, d, e, f $\in$ N $\\$ We have$\\$ [(a, b) * (c, d)] * (e, f) = (a + c, b + d) * (e, f) = (a + c + e, b + d + f) and $\\$ (a, b) * [(c, d) * (e, f)] = (a, b) * (c + e, d + f) = (a + c + e, b + d + f)$\\$ $\therefore$ [(a, b) * (c, d)] * (e, f) = (a, b) * [(c, d) * (e, f)]$\\$ Therefore, the operation * is associative.$\\$ Let an element $e = (e_ 1 , e_ 2 ) \in A$ will be an identity $\\$element for the operation * if a * e = a = e * a$\\$ for all a = $(a_ 1 , a _2 ) \in A$$\\ i.e., (a_ 1 + e _1 , a_ 2 + e_ 2 ) = (a_ 1 , a_ 2 ) = (e_ 1 + a_ 1 , e_ 2 + a_ 2 )$$\\$ Which is not true for any element in A.$\\$ Therefore, the operation * does not have any identity element.

50   State whether the following statements are true or false. Justify.$\\$ (i) For an arbitrary binary operation * on a set N, a * a = a $\forall$ a $\in$ N.$\\$ (ii) If * is a commutative binary operation on N, then a * (b * c) = (c * b) * a

(i) Define an operation * on N as a * b = a + b $\forall$ a, b $\in$ N $\\$ Then, in particular, for b = a = 3, we have$\\$ 3 * 3 = 3 + 3 = 6 $\neq$ 3$\\$ Therefore, statement (i) is false.$\\$$\\ (ii) R.H.S. = (c * b) * a\\ = (b * c) * a [* is commutative]\\ = a * (b * c) [Again, as * is commutative]\\ = L.H.S.\\ \therefore a * (b * c) = (c * b) * a\\ Therefore, statement (ii) is true 51 Consider a binary operation * on N defined as a * b = a^ 3 + b^ 3 . Choose the correct answer.\\ ##### Solution : On N, the operation * is defined as a * b = a ^3 + b^ 3 .\\ For, a, b, \in N, we have\\ a * b = a^ 3 + b^ 3 = b ^3 + a^ 3 = b * a$$\\$ [Addition is commutative in N]$\\$ Therefore, the operation * is commutative.$\\$ It can be observed that$\\$ $(1 * 2) * 3 = (1^ 3 + 2 ^3 ) * 3 = (1 + 8) * 3 = 9 * 3 = 9^ 3 + 3^ 3$$\\ = 729 + 27 = 756 and\\ 1* (2 * 3) = 1 * (2 ^3 + 3^ 3 ) = 1* (8 + 27) = 1 * 35 = 1 ^3 + 35 ^3 = 1 + 42875 = 42876\\ \therefore (1 * 2) * 3 \neq 1 * (2 * 3), where 1, 2, 3 \in N\\ Therefore, the operation * is not associative.\\ Hence, the operation * is commutative, but not associative. Thus, the correct answer is B. 52 Let f: R \to R be defined as f(x) = 10x + 7. Find the function g: R \to R such that gof = fog = I_ R . ##### Solution : 53 If f: R \to R is defined by f(x) = x^ 2 - 3x + 2, find f(f(x)). ##### Solution : It is given that f: R \to R is defined as f(x) = x ^2 - 3x + 2.$$\\$ $(f(x)) = f(x^ 2 - 3x + 2)\\ = (x ^2 -3x + 2)^ 2 - 3(x ^2 - 3x + 2) + 2\\ = (x ^4 + 9x ^2 + 4 - 6x^ 3 -12x + 4x ^2 ) + (-3x^ 2 + 9x - 6) + 2\\ = x^ 4 - 6x ^3 + 10x ^2 - 3x$

54   Show that the function $f: R \to R$ given by $f(x) = x ^3$ is injective.

##### Solution :

$f: R \to R$ is given as $f(x) = x ^3 .$$\\ For one - one\\ Suppose f(x) = f(y), where x, y \in R.$$\\$ $x ^3 = y ^3 (1)$$\\ Now, we need to show that x = y. Suppose x \neq y, their cubes will also not be equal. x^ 3 \neq y^ 3$$\\$ However, this will be a contradiction to (1).$\\$ $\therefore x = y$ Hence,$f$ is injective

55   Give examples of two functions $f: N \to Z$ and $g: Z \to Z$ such that $gof$ is injective but $g$ is not injective.$\\$ (Hint: Consider $f(x) = x$ and $g (x) = |x|)$

Define $f: N \to Z$ as $f(x) = x$ and $g: Z \to Z$ as $g (x) = |x|.$$\\ We first show that g is not injective.\\ It can be observed that\\ (-1) =|-1| = 1\\ (1) = |1| = 1\\ \therefore (-1) = g(1), but -1 \neq 1.$$\\$ $\therefore g$ is not injective.$\\$ Now, g of: $N \to Z$ is defined as $go(x) = g(f(x)) = g(x) = |x|.$$\\ Let x, y \in N such that gof(x) = gof(y).$$\\$ $|x| = |y|$$\\ Since x and y \in N, both are positive.\\ \therefore |x| = |y| \implies x = y Hence, gof is injective. 56 Given a non-empty set X, consider P(X) which is the set of all subsets of X. Define the relation R in P(X) as follows:\\ For subsets A, B in P(X), ARB if and only if A \subset B. Is R an equivalence relation on P(X)? Justify you answer ##### Solution : Since every set is a subset of itself, ARA for all A \in P(X).\\ \therefore R is reflexive.\\ LetARB \implies A \subset B.$$\\$ This cannot be implied to $B \subset A.$$\\ For instance, if A = \{1, 2\} and B = \{1, 2, 3\}, then it cannot be implied that B is related to A.\\ \therefore R is not symmetric.\\ Further, if ARB and BRC, then A \subset B and B \subset C.\\ A \subset C$$\\$ $ARC$ $\therefore R$ is transitive.$\\$ Hence, $R$ is not an equivalence relation as it is not symmetric.

57   Given a non-empty set $X$, consider the binary operation $*: P(X) × P(X) \to P(X)$ given by $A*B = A \cap B \forall A, B$ in $P(X)$ is the power set of $X$. Show that $X$ is the identity element for this operation and X is the only invertible element in $P(X)$ with respect to the operation*.

##### Solution :

It is given the binary operation *:$\\$ $P(X) × P(X) \to P(X)$ given by $A * B = A \cap B \forall A, B$in $P(X)$$\\ We know that A\cap X = A = X \cap A for all A \in P(X)$$\\$ $A * X = A = X * A$ for all $A \in P(X)$$\\ Thus, X is the identity element for the given binary operation *.\\ Now, an element A \in P(X) is invertible if there exists B \in P(X) such that\\ A * B = X = B * A$$\\$ [As X is the identity element]$\\$ or$\\$ $A \cap B = X = B \cap A$ 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.

58   Find the number of all onto functions from the set $\{1, 2, 3, ... , n\}$ to itself.

Onto functions from the set $\{1, 2, 3, ... , n\}$ to itself is simply a permutation on $n$ symbols $1, 2, ..., n.$$\\ Thus, the total number of onto maps from \{1, 2, ... , n\} to itself is the same as the total number of permutations on n symbols 1, 2, ..., n, which is n. 59 Let S = \{a, b, c\} and T = \{1, 2, 3\}. Find F -1 of the following functions F from S to T, if it exists.\\ (i) F = \{(a, 3), (b, 2), (c, 1)\}\\ (ii) F = \{(a, 2), (b, 1), (c, 1)\} ##### Solution : S = \{a, b, c\}, T = \{1, 2, 3\}\\ (i) F: S \to T is defined as F = \{(a, 3), (b, 2), (c, 1)\}\\ F (a) = 3, F (b) = 2, F(c) = 1$$\\$ Therefore, $F-1: T \to S$ is given by $F -1 = \{(3, a), (2, b), (1, c)\}$.$\\$ $(ii) F: S \to T$ is defined as $F = \{(a, 2), (b, 1), (c, 1)\}$$\\ Since F (b) = F (c) = 1, F is not one - one.\\ Hence, F is not invertible i.e., F -1 does not exist. 60 Consider the binary operations *: R × R \to R and o: R × R \to R defined as a * b = |a - b| and a o b = a, \forall a, b \in R. Show that * is commutative but not associative, o is associative but not commutative. Further, show that \forall a, b, c \in R, a*(b o c) = (a * b) o (a * c). [If it is so, we say that the operation * distributes over the operation o]. Does o distribute over *? Justify your answer. ##### Solution : It is given that *: R × R \to and o: R × R \to R is defined as a * b = |a - b| and aob = a, \forall a, b \in R$$\\$ For $a, b \in R$, we have $a * b = |a - b|$ and $b *a = |b - a|\\ = |-(a - b)| = |a - b| \\ \therefore a * b = b * a$$\\ Hence, the operation * is commutative.\\ It can be observed that\\ (1 * 2) * 3 = (|1 - 2|) * 3 = 1 * 3 = |1 - 3| = 2$$\\$ and $\\$ $1 * (2 * 3) = 1 * (|2 - 3|) = 1 * 1 = |1 - 1| = 0\\ \therefore (1 * 2) * 3 \neq 1 * (2 * 3)$ where$1, 2, 3 \in R$.$\\$ Hence, the operation * is not associative.$\\$ Now, consider the operation o$\\$ It can be observed that 1 o 2 = 1 and 2 o 1 = 2.$\\$ $\therefore 1 o 2 \neq 2 o 1$ where 1, 2 $\in$ R.$\\$ Hence, the operation o is not commutative.$\\$ Let a, b, c $\in$ R. Then, we have$\\$ $(a o b) o c = a o c = a$$\\ and\\ a o (b o c) = a o b = a \therefore (a o b) o c = a o (b o c), where a, b, c \in R$$\\$ Hence, the operation o is associative.$\\$ Now, let a, b, c $\in$ R, then we have$\\$ $a * (b o c) = a * b =|a - b|\\ (a * b) o (a * c) = (|a - b|)o(|a - c|) = |a - b|$$\\ Hence, a * (b o c) = (a * b) o (a * c).$$\\$ Now,$\\$ $1 o (2 * 3) = 1 o (|2 - 3|) = 1 o 1 = 1\\ (1 o 2) * (1 o 3) = 1 * 1 = |1 - 1| = 0\\ \therefore 1 o (2 * 3) \neq (1 o 2) * (1 o 3)$ where $1, 2, 3 \in R$ Hence, the operation o does not distribute over *

61   Let $A = \{1, 2, 3\}$. Then number of relations containing $(1, 2)$ and $(1, 3)$ which are reflexive and symmetric but not transitive is

##### Solution :

The given set is $A = \{1, 2, 3\}$.$\\$ The smallest relation containing $(1, 2)$ and $(1, 3)$ which is reflexive and symmetric, but not transitive is given by:$\\$ $R = \{(1, 1), (2, 2), (3, 3), (1, 2), (1, 3), (2, 1), (3, 1)\}$$\\ This is because relation R is reflexive as (1, 1), (2, 2), (3, 3) \in R.\\ Relation R is symmetric since (1, 2), (2, 1) \in R and (1, 3), (3, 1) \in R. But relation R is not transitive as (3, 1), (1, 2) \in R, but (3, 2) \notin R.$$\\$ Now, if we add any two pairs $(3, 2)$ and $(2, 3)$ (or both) to relation $R$, then relation $R$ will become transitive.$\\$ Hence, the total number of desired relations is one.$\\$ The correct answer is $A$.

62   Let $A = \{1, 2, 3\}$. Then number of equivalence relations containing $(1, 2)$ is