# Class 12 RD Sharma Solutions – Chapter 1 Relations – Exercise 1.2 | Set 1

Last Updated : 09 Nov, 2022

### Question 1. Show that the relation R = {(a,b): a-b is divisible by 3;, a, b âˆˆ Z} is an equivalence relation.

Solution:

According to question, relation R = {(a,b): a-b is divisible by 3;, a, b âˆˆ Z}

We have to show that R is an equivalence relation.

(i) reflexibity:

let a = z

=> a – a = 0

=> 0 is divisible by 3

:. a is divisible by 3

(a, a) âˆˆ R,

Hence, relation R is reflexive.

(ii) symmetry:

Let a, bâˆˆ Z and (a, b) âˆˆ R

=> a – b is divisible by 3

=> a – b = 3x, for some x âˆˆ Z

=> b -a = 3 (-x)

here, -x âˆˆ Z

:. b – a is divisible by 3

Hence, (b- a) âˆˆ R for all a, b âˆˆ Z

So, relation R is symmetric.

(iii) transitivity:

let (a, b) and (b, c) âˆˆ R

a – b and b – c is divisible by 3

a – b = 3x, for some x âˆˆ Z & (eqn. 1)

b – c = 3y, for some y âˆˆ Z (eqn. 2)

Adding the above equations 1 and 2, we get

a – c = 3(x + y)

here, x and y âˆˆ Z

so, a – c is divisible by 3

Hence, a – c âˆˆ R for all a, c âˆˆ Z

thus relation R is transitive.

As we know if a relation is reflexive, symmetric and transitive at the same time, then it is called an equivalence relation.

Therefore, Relation R is an equivalence relation.

### Question 2. Show that the relation R on the set of integers, given by R = {(a, b): 2 divides a – b}, is an equivalence relation.

Solution:

Given, R = {(a, b): 2 divides a – b} which is defined on Z

We have to show that relation R is an equivalence relation

(i) reflexibility :

let na be an arbitrary element on set Z

then a âˆˆ  R

a – a = 0

=> 0 X 2 = 0

2 divides a – a

=> (a, a) âˆˆ  R

:. Relation R is a reflexive relation.

Now, (ii) symmetric :

let (a,  b) âˆˆ  R

2 divides a – b

a – b = 2x for some x âˆˆ  Z

b – a = 2 (- x) where – x âˆˆ Z

2 divides b – a

=> (b, a) âˆˆ  R

and  (a, b) âˆˆ  R

therefore, relation R is symmetric.

(iii) transitivity :

let a, b, c âˆˆ  Z such that (a, b) âˆˆ  R and (b, c) âˆˆ  R

then, (a, b) âˆˆ  R => 2 divides b – a

b – a = 2x for some x âˆˆ  Z  (eqn.1)

and(b – c) âˆˆ  R

2 divides c – b => c – b = 2y for some y âˆˆ  Z  (eqn 2)

on solving equation 1 and 2,

c – a = 2 (x + y) where x + y âˆˆ  Z

2 divides c – a

(a, c) âˆˆ R

Therefore, relation R is transitive.

Hence, relation R is an equivalence relation.

### Question 3. Prove that the relation R on Z defined by (a, b) âˆˆ R <=> a – b is divisible by 5 is an equivalence relation on Z.

Solution:

Given, relation R on Z defined by (a, b) âˆˆ R <=> a – b is divisible by 5

as, we have to prove it a equivalence relation, the relation R must have to be reflexive, symmetric as well as transitive.

(i) Reflexibility :

Let a be an arbitrary element of R

=> a – a = 0

=> 0 is divisible by 5

=> a – a is divisible by 5

=> (a, a) âˆˆ R for all a âˆˆ Z

:. relation R is reflexive.

Again, (ii) symmetry

Let (a, b) âˆˆ R

=> a âˆ’ b is divisible by 5

=> a âˆ’ b = 5x for some x âˆˆ Z  , b âˆ’ a = 5 (âˆ’x)

since (âˆ’x) âˆˆ Z

b âˆ’ a is divisible by 5

(b, a) âˆˆ R for all a, b âˆˆ Z

So, relation R is symmetric.

(iii) transitivity ;

Let (a, b) and (b, c) âˆˆ R

=> a âˆ’ b is divisible by 5

=>a âˆ’ b = 5x for some x âˆˆ Z (eqn. 1)

Also, b âˆ’ c is divisible by 5

=> b âˆ’ c = 5y for some y âˆˆ Z  (eqn. 2)

a âˆ’b + b âˆ’ c = 5x + 5y

=> a âˆ’ c = 5 (x + y)

=> a âˆ’ c is divisible by 5

Since, x + y âˆˆ Z

=>(a, c) âˆˆ R for all a, c âˆˆ Z

So, R is transitive on Z.

As relation R is reflexive, symmetric and transitive,

Hence, R is an equivalence relation on Z.

### Question 4. Let n be a fixed positive integer. Define a relation R on Z as follows : (a, b) âˆˆ R <=> a – b is divisible by n.

Solution:

Given (a, b) âˆˆ R â‡” a âˆ’ b is divisible by n is a relation R defined on Z.

it is necessary that the relation R should be reflexive, symmetric and transitive.

(i) Reflexivity:

Let a âˆˆ N

a âˆ’ a = 0

= 0 Ã— n

=> a âˆ’ a is divisible by n

=> (a, a) âˆˆ R

=> (a, a) âˆˆ R for all a âˆˆ Z

:.R is reflexive on Z.

(ii) Symmetry:

Let (a, b) âˆˆ R

a âˆ’ b is divisible by n

=> a âˆ’ b = nx for some x âˆˆ Z

=> b âˆ’ a = n (âˆ’x)

=> b âˆ’ a is divisible by n

=> (b, a) âˆˆ R

So, relation R is symmetric on Z.

Similarly (iii) Transitivity:

Let (a, b) and (b, c) âˆˆ R

a âˆ’ b is divisible by n and b âˆ’ c is divisible by n.

=> a âˆ’ b= n x for some x âˆˆ Z  (eqn . 1)

And bâˆ’c = ny for some y âˆˆ Z   (eqn. 2)

a â€“ b + b âˆ’ c = nx + n y

=> a âˆ’ c = n (p + q)

=> (a, c) âˆˆ R for all a, c âˆˆ Z

So, relation R is transitive on Z.

Therefore, R is reflexive, symmetric and transitive.

Hence, R is an equivalence relation on Z.

### Question 5. Let Z be the set of integers. Show that the relation R  = {(a, b): a, b âˆˆ Z and a + b is even} is an equivalence relation on Z.

Solution:

Given R = {(a, b): a, b âˆˆ Z and a + b is even} is a relation defined on R.

Also given that Z be the set of integers

it is necessary that the given relation should be reflexive, symmetric and transitive.

Reflexivity:

Let a be an arbitrary element of Z.

Then, a âˆˆ R

a + a = 2a is even for all a âˆˆ Z.

=> (a, a) âˆˆ R for all a âˆˆ Z

So, R is reflexive on Z.

(ii) Symmetry:

Let (a, b) âˆˆ R

=> a + b is even

=> b + a is even

=>(b, a) âˆˆ R for all a, b âˆˆ Z

So, R is symmetric on Z.

(iii) Transitivity:

Let (a, b) and (b, c) âˆˆ R

=> a + b and b + c are even

let a + b = 2x for some x âˆˆ Z  (eqn. 1)

And b + c = 2y for some y âˆˆ Z (eqn. 2)

Adding the above two equations, we get

A + 2b + c = 2x + 2y

=> a + c = 2 (x + y âˆ’ b), which is even for all x, y, b âˆˆ Z

Thus, (a, c) âˆˆ R

So, R is transitive on Z.

Therefore, relation R is reflexive, symmetric and transitive.

Hence, R is an equivalence relation on Z

6. m is said to be related to n if m and n are integers and m âˆ’ n is divisible by 13. Does this define an equivalence relation?

### Question 6. m is said to be related to n if m and n are integers and m âˆ’ n is divisible by 13. Does this define an equivalence relation?

Solution:

Given that m is said to be related to n if m and n are integers and m âˆ’ n is divisible by 13

So, we have to check whether the given relation is equivalence or not.

Let R = {(m, n): m, n âˆˆ Z : m âˆ’ n is divisible by 13}

Reflexivity:

Let m be an arbitrary element of Z.

Then, m âˆˆ R

=> m âˆ’ m = 0 = 0 Ã— 13

=> m âˆ’ m is divisible by 13

=> (m, m) is reflexive on Z.

Now, Symmetry:

Let (m, n) âˆˆ R.

Then, m âˆ’ n is divisible by 13

=> m âˆ’ n = 13p

Here, p âˆˆ Z

=>n â€“ m = 13 (âˆ’p)

=> n âˆ’ m is divisible by 13

=>(n, m) âˆˆ R for all m, n âˆˆ Z

So, R is symmetric on Z.

Transitivity:

Let (m, n) and (n, o) âˆˆ R

=>m âˆ’ n and n âˆ’ o are divisible by 13  (eqn. 1)

=>m â€“ n = 13p and n âˆ’ o = 13q for some p, q âˆˆ Z  (eqn.2)

Adding the above two equations, we get

=>m â€“ n + n âˆ’ o = 13p + 13q

=> mâˆ’o = 13 (p + q)

=> m âˆ’ o is divisible by 13

=>(m, o) âˆˆ R for all m, o âˆˆ Z

So, R is transitive on Z.

Therefore, R is reflexive, symmetric and transitive.

Hence, R is an equivalence relation on Z.

### Question 7. Let R be a relation on the set A of ordered pair of integers defined by (x, y) R (u, v) if xv = y u. Show that R is an equivalence relation.

Solution:

First let R be a relation on A

It is given that set A of ordered pair of integers defined by (x, y) R (u, v) if xv = y u

We have to check whether the given relation is equivalence or not.

Reflexivity:

Let (a, b) be an arbitrary element of the set A.

Then, (a, b) âˆˆ A

=> a b = b a

=>(a, b) R (a, b)

Thus, R is reflexive on A.

Again, Symmetry:

Let (x, y) and (u, v) âˆˆ A such that (x, y) R (u, v). Then,

x v = y u

=> v x = u y

=> u y = v x

=>(u, v) R (x, y)

So, R is symmetric on A.

Transitivity:

Let (x, y), (u, v) and (p, q) âˆˆR such that (x, y) R (u, v) and (u, v) R (p, q)

=> x v = y u and u q = v p

Multiplying the corresponding sides, we get

x v Ã— u q = y u Ã— v p

=> x q = y p

=>(x, y) R (p, q)

So, R is transitive on A.

Therefore, R is reflexive, symmetric and transitive.

Hence, R is an equivalence relation on A.

### Question 8. Show that the relation R on the set A = {x âˆˆ Z; 0 â‰¤ x â‰¤ 12}, given by R = {(a, b): a = b}, is an equivalence relation. Find the set of all elements related to 1.

Solution:

According to question, set A = {x âˆˆ Z; 0 â‰¤ x â‰¤ 12}

Also given that relation R = {(a, b): a = b} is defined on set A

We have to find whether the given relation is equivalence or not.

(i) Reflexivity:

Let a be an arbitrary element of A.

Then, a âˆˆ R

=>a = a (because, every element is equal to itself)

=> (a, a) âˆˆ R for all a âˆˆ A

:. R is reflexive on A.

(ii) Symmetry:

Let (a, b) âˆˆ R

=>b = a

=> (b, a) âˆˆ R for all a, b âˆˆ A

So, R is symmetric on A.

(iii) Transitivity:

Let (a, b) and (b, c) âˆˆ R

=> a =b and b = c

=>a = b c

=>a = c

=> (a, c) âˆˆ R

So, R is transitive on A.

Hence, relation R is an equivalence relation on A.

Therefore, relation R is reflexive, symmetric and transitive.

The set of all elements related to 1 is {1}.

### Question 9. Let L be the set of all lines in XY – plane and R be the relation in L defined as R = {(L1, L2) : L1 is parallel to L2}. Show that R is an equivalence relation. Find the set of all lines related to the line y = 2x + 4.

Solution:

Given, L is the set of lines.

R = {(L1, L2) : L1 is parallel to L2} be a relation on L

Now, (i) reflexibility :

as we know, a line is always parallel to itself,

So, L1, L2 âˆˆ R

Therefore, R is reflexive.

Again, (ii) symmetry :

Assume, L1, L2 âˆˆ  L and (L1, L2) âˆˆ R

Since, L1 is parallel to L2

:. L2 is also parallel to L1

Thus, relation R is symmetric.

(iii) transitivity :

let L1, L2, L3 âˆˆ R in such a way that (L1, L2) âˆˆ R and (L2, L3) âˆˆ R

L1 is parallel to L2

L2 is parallel to L

Therefore L1 is parallel to L3

Hence, relation R is transitive.

So, relation R is an equivalence relation.

Now, the set of all lines related to line y = 2x + 4 is y = 2x + c for all c âˆˆ R.

### Question 10. Show that the relation R, defined on the set A of all polygons as R = {(P1, P2):P1 and P2 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:

The relation R is defined as R = {(P1, P2):P1 and P2 have the same number of sides}

(i) reflexibility:

Let P be any polygon in A

Then, P and P have same number of sides

(P, P) âˆˆ R

Thus, relation R is reflexive.

(ii) symmetry:

Let P1 and P2 be any polygon in A such that, (P1, P2 ) âˆˆ R

(P1, P2) âˆˆ R

P1 and P2 have same number of sides

so, P2 and P1 will have also same number of sides

Hence (P1, P2) âˆˆ R

so, relation R is symmetric.

at last (iii) transitivity :

let P1, P2, P3 be three polygons in A in such a way that (P1 , P2) âˆˆ R and (P2, P3) âˆˆ R

then, (P1, P2) have same number of sides

(P2, P3) have same number of sides

:. P1 and P3 will also have same number of sides.

Hence, relation R is transitive

so, relation R is an equivalence relation.

Now, let P be a polygon in A such that (P, T) âˆˆ R, where T is a right angle triangle with sides 3, 4 and 5

Then, (P, T) âˆˆ R

polygon P and triangle T have same number of sides

Hence, the set of all elements in A related to T is the set of all triangles in A.

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