NCERT Solutions Chapter 2 – Relations and Functions Class 11

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Exercise 2.1

Question 1:

If , find the values of x and y.

Answer

It is given that .

Since the ordered pairs are equal, the corresponding elements will also be equal.

Therefore, and .

∴ x = 2 and y = 1

Question 2:

If the set A has 3 elements and the set B = {3, 4, 5}, then find the number of elements

in (A ~ B)?

Answer

It is given that set A has 3 elements and the elements of set B are 3, 4, and 5.

⇒ Number of elements in set B = 3

Number of elements in (A × B)

= (Number of elements in A) × (Number of elements in B)

= 3 × 3 = 9

Thus, the number of elements in (A × B) is 9.

Question 3:

If G = {7, 8} and H = {5, 4, 2}, find G ~ H and H ~ G.

Answer

G = {7, 8} and H = {5, 4, 2}

We know that the Cartesian product P ~ Q of two non-empty sets P and Q is defined as

P ~ Q = {(p, q): p∈ P, q ∈ Q}

∴G × H = {(7, 5), (7, 4), (7, 2), (8, 5), (8, 4), (8, 2)}

H × G = {(5, 7), (5, 8), (4, 7), (4, 8), (2, 7), (2, 8)}

Question 4:

State whether each of the following statement are true or false. If the statement is false,

rewrite the given statement correctly.

Class XI Chapter 2 – Relations and Functions Maths

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(i) If P = {m, n} and Q = {n, m}, then P ~ Q = {(m, n), (n, m)}.

(ii) If A and B are non-empty sets, then A ~ B is a non-empty set of ordered pairs (x, y)

such that x ∈ A and y ∈ B.

(iii) If A = {1, 2}, B = {3, 4}, then A × (B ∩ Φ) = Φ.

Answer

(i) False

If P = {m, n} and Q = {n, m}, then

P ~ Q = {(m, m), (m, n), (n, m), (n, n)}

(ii) True

(iii) True

Question 5:

If A = {–, 1}, find A × A × A.

Answer

It is known that for any non-empty set A, A × A × A is defined as

A × A × A = {(a, b, c): a, b, c ∈ A}

It is given that A = {–, 1}

∴ A × A × A = {(–, –, –), (–, –, 1), (–, 1, –), (–, 1, 1),

(1, –, –), (1, –, 1), (1, 1, –), (1, 1, 1)}

Question 6:

If A ~ B = {(a, x), (a, y), (b, x), (b, y)}. Find A and B.

Answer

It is given that A ~ B = {(a, x), (a, y), (b, x), (b, y)}

We know that the Cartesian product of two non-empty sets P and Q is defined as P ~ Q

= {(p, q): p ∈ P, q ∈ Q}

∴ A is the set of all first elements and B is the set of all second elements.

Thus, A = {a, b} and B = {x, y}

Question 7:

Let A = {1, 2}, B = {1, 2, 3, 4}, C = {5, 6} and D = {5, 6, 7, 8}. Verify that

(i) A ~ (B ∩ C) = (A × B) ∩ (A × C)

(ii) A × C is a subset of B × D

Answer

(i) To verify: A × (B ∩ C) = (A × B) ∩ (A × C)

We have B ∩ C = {1, 2, 3, 4} ∩ {5, 6} = Φ

∴L.H.S. = A × (B ∩ C) = A × Φ = Φ

A × B = {(1, 1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3), (2, 4)}

A × C = {(1, 5), (1, 6), (2, 5), (2, 6)}

∴ R.H.S. = (A × B) ∩ (A × C) = Φ

∴L.H.S. = R.H.S

Hence, A × (B ∩ C) = (A × B) ∩ (A × C)

(ii) To verify: A × C is a subset of B × D

A × C = {(1, 5), (1, 6), (2, 5), (2, 6)}

B × D = {(1, 5), (1, 6), (1, 7), (1, 8), (2, 5), (2, 6), (2, 7), (2, 8), (3, 5), (3, 6), (3, 7),

(3, 8), (4, 5), (4, 6), (4, 7), (4, 8)}

We can observe that all the elements of set A × C are the elements of set B × D.

Therefore, A × C is a subset of B × D.

Class XI Chapter 2 –Relations and Functions Maths

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Question 8:

Let A = {1, 2} and B = {3, 4}. Write A ~ B. How many subsets will A ~ B have? List

them.

Answer

A = {1, 2} and B = {3, 4}

∴A × B = {(1, 3), (1, 4), (2, 3), (2, 4)}

⇒ n(A ~ B) = 4

We know that if C is a set with n(C) = m, then n[P(C)] = 2m.

Therefore, the set A ~ B has 24 = 16 subsets. These are

Φ, {(1, 3)}, {(1, 4)}, {(2, 3)}, {(2, 4)}, {(1, 3), (1, 4)}, {(1, 3), (2, 3)},

{(1, 3), (2, 4)}, {(1, 4), (2, 3)}, {(1, 4), (2, 4)}, {(2, 3), (2, 4)},

{(1, 3), (1, 4), (2, 3)}, {(1, 3), (1, 4), (2, 4)}, {(1, 3), (2, 3), (2, 4)},

{(1, 4), (2, 3), (2, 4)}, {(1, 3), (1, 4), (2, 3), (2, 4)}

Question 9:

Let A and B be two sets such that n(A) = 3 and n (B) = 2. If (x, 1), (y, 2), (z, 1) are in A

~ B, find A and B, where x, y and z are distinct elements.

Answer

It is given that n(A) = 3 and n(B) = 2; and (x, 1), (y, 2), (z, 1) are in A ~ B.

We know that A = Set of first elements of the ordered pair elements of A ~ B

B = Set of second elements of the ordered pair elements of A ~ B.

∴ x, y, and z are the elements of A; and 1 and 2 are the elements of B.

Since n(A) = 3 and n(B) = 2, it is clear that A = {x, y, z} and B = {1, 2}.

Question 10:

The Cartesian product A ~ A has 9 elements among which are found (–, 0) and (0, 1).

Find the set A and the remaining elements of A × A.

Answer

We know that if n(A) = p and n(B) = q, then n(A ~ B) = pq.

∴ n(A ~ A) = n(A) ~ n(A)

It is given that n(A ~ A) = 9

∴ n(A) ~ n(A) = 9

⇒ n(A) = 3

The ordered pairs (–, 0) and (0, 1) are two of the nine elements of A × A.

We know that A × A = {(a, a): a ∈ A}. Therefore, –, 0, and 1 are elements of A.

Since n(A) = 3, it is clear that A = {–, 0, 1}.

The remaining elements of set A × A are (–, –), (–, 1), (0, –), (0, 0),

(1, –), (1, 0), and (1, 1)

Class XI Chapter 2 –Relations and Functions Maths

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Exercise 2.2

Question 1:

Let A = {1, 2, 3, … , 14}. Define a relation R from A to A by R = {(x, y): 3x –y = 0,

where x, y ∈ A}. Write down its domain, codomain and range.

Answer

The relation R from A to A is given as

R = {(x, y): 3x –y = 0, where x, y ∈ A}

i.e., R = {(x, y): 3x = y, where x, y ∈ A}

∴R = {(1, 3), (2, 6), (3, 9), (4, 12)}

The domain of R is the set of all first elements of the ordered pairs in the relation.

∴Domain of R = {1, 2, 3, 4}

The whole set A is the codomainof the relation R.

∴Codomain of R = A = {1, 2, 3, …, 14}

The range of R is the set of all second elements of the ordered pairs in the relation.

∴Range of R = {3, 6, 9, 12}

Question 2:

Define a relation R on the set N of natural numbers by R = {(x, y): y = x + 5, x is a

natural number less than 4; x, y ∈ N}. Depict this relationship using roster form. Write

down the domain and the range.

Answer

R = {(x, y): y = x + 5, x is a natural number less than 4, x, y ∈ N}

The natural numbers less than 4 are 1, 2, and 3.

∴R = {(1, 6), (2, 7), (3, 8)}

The domain of R is the set of all first elements of the ordered pairs in the relation.

∴ Domain of R = {1, 2, 3}

The range of R is the set of all second elements of the ordered pairs in the relation.

∴ Range of R = {6, 7, 8}

Question 3:

A = {1, 2, 3, 5} and B = {4, 6, 9}. Define a relation R from A to B by R = {(x, y): the

difference between x and y is odd; x ∈ A, y ∈ B}. Write R in roster form.

Answer

A = {1, 2, 3, 5} and B = {4, 6, 9}

R = {(x, y): the difference between x and y is odd; x ∈ A, y ∈ B}

∴R = {(1, 4), (1, 6), (2, 9), (3, 4), (3, 6), (5, 4), (5, 6)}

Question 4:

The given figure shows a relationship between the sets P and Q. write this relation

(i) in set-builder form (ii) in roster form.

What is its domain and range?

Class XI Chapter 2 –Relations and Functions Maths

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Answer

According to the given figure, P = {5, 6, 7}, Q = {3, 4, 5}

(i) R = {(x, y): y = x –2; x ∈ P} or R = {(x, y): y = x –2 for x = 5, 6, 7}

(ii) R = {(5, 3), (6, 4), (7, 5)}

Domain of R = {5, 6, 7}

Range of R = {3, 4, 5}

Question 5:

Let A = {1, 2, 3, 4, 6}. Let R be the relation on A defined by

{(a, b): a, b ∈ A, b is exactly divisible by a}.

(i) Write R in roster form

(ii) Find the domain of R

(iii) Find the range of R.

Answer

A = {1, 2, 3, 4, 6}, R = {(a, b): a, b ∈ A, b is exactly divisible by a}

(i) R = {(1, 1), (1, 2), (1, 3), (1, 4), (1, 6), (2, 2), (2, 4), (2, 6), (3, 3), (3, 6), (4, 4),

(6, 6)}

(ii) Domain of R = {1, 2, 3, 4, 6}

(iii) Range of R = {1, 2, 3, 4, 6}

Question 6:

Determine the domain and range of the relation R defined by R = {(x, x + 5): x ∈ {0, 1,

2, 3, 4, 5}}.

Answer

R = {(x, x + 5): x ∈ {0, 1, 2, 3, 4, 5}}

∴ R = {(0, 5), (1, 6), (2, 7), (3, 8), (4, 9), (5, 10)}

∴Domain of R = {0, 1, 2, 3, 4, 5}

Range of R = {5, 6, 7, 8, 9, 10}

Question 7:

Write the relation R = {(x, x3): x is a prime number less than 10} in roster form.

Answer

R = {(x, x3): x is a prime number less than 10}

The prime numbers less than 10 are 2, 3, 5, and 7.

∴R = {(2, 8), (3, 27), (5, 125), (7, 343)}

Question 8:

Let A = {x, y, z} and B = {1, 2}. Find the number of relations from A to B.

Answer

It is given that A = {x, y, z} and B = {1, 2}.

∴ A × B = {(x, 1), (x, 2), (y, 1), (y, 2), (z, 1), (z, 2)}

Since n(A ~ B) = 6, the number of subsets of A ~ B is 26.

Class XI Chapter 2 –Relations and Functions Maths

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Therefore, the number of relations from A to B is 26.

Question 9:

Let R be the relation on Z defined by R = {(a, b): a, b ∈ Z, a –b is an integer}. Find the

domain and range of R.

Answer

R = {(a, b): a, b ∈ Z, a –b is an integer}

It is known that the difference between any two integers is always an integer.

∴Domain of R = Z

Range of R = Z

Class XI Chapter 2 –Relations and Functions Maths

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Exercise 2.3

Question 1:

Which of the following relations are functions? Give reasons. If it is a function, determine

its domain and range.

(i) {(2, 1), (5, 1), (8, 1), (11, 1), (14, 1), (17, 1)}

(ii) {(2, 1), (4, 2), (6, 3), (8, 4), (10, 5), (12, 6), (14, 7)}

(iii) {(1, 3), (1, 5), (2, 5)}

Answer

(i) {(2, 1), (5, 1), (8, 1), (11, 1), (14, 1), (17, 1)}

Since 2, 5, 8, 11, 14, and 17 are the elements of the domain of the given relation having

their unique images, this relation is a function.

Here, domain = {2, 5, 8, 11, 14, 17} and range = {1}

(ii) {(2, 1), (4, 2), (6, 3), (8, 4), (10, 5), (12, 6), (14, 7)}

Since 2, 4, 6, 8, 10, 12, and 14 are the elements of the domain of the given relation

having their unique images, this relation is a function.

Here, domain = {2, 4, 6, 8, 10, 12, 14} and range = {1, 2, 3, 4, 5, 6, 7}

(iii) {(1, 3), (1, 5), (2, 5)}

Since the same first element i.e., 1 corresponds to two different images i.e., 3 and 5,

this relation is not a function.

Question 2:

Find the domain and range of the following real function:

(i) f(x) = –x| (ii)

Answer

(i) f(x) = –x|, x ∈ R

We know that |x| =

Since f(x) is defined for x ∈ R, the domain of f is R.

It can be observed that the range of f(x) = –x| is all real numbers except positive real

numbers.

∴The range of f is (–, 0].

(ii)

Since is defined for all real numbers that are greater than or equal to – and less

than or equal to 3, the domain of f(x) is {x : – ≤x ≤3} or [–, 3].

For any value of x such that – ≤x ≤3, the value of f(x) will lie between 0 and 3.

∴The range of f(x) is {x: 0 ≤x ≤3} or [0, 3].

Question 3:

A function f is defined by f(x) = 2x –5. Write down the values of

(i) f(0), (ii) f(7), (iii) f(–)

Class XI Chapter 2 –Relations and Functions Maths

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Answer

The given function is f(x) = 2x –5.

Therefore,

(i) f(0) = 2 ~ 0 –5 = 0 –5 = –

(ii) f(7) = 2 ~ 7 –5 = 14 –5 = 9

(iii) f(–) = 2 × (–) –5 = –6 –5 = –1

Question 4:

The function ‘t’ which maps temperature in degree Celsius into temperature in degree

Fahrenheit is defined by .

Find (i) t (0) (ii) t (28) (iii) t (–0) (iv) The value of C, when t(C) = 212

Answer

The given function is .

Therefore,

(i)

(ii)

(iii)

(iv) It is given that t(C) = 212

Thus, the value of t, when t(C) = 212, is 100.

Question 5:

Find the range of each of the following functions.

(i) f(x) = 2 –3x, x ∈ R, x > 0.

(ii) f(x) = x2 + 2, x, is a real number.

(iii) f(x) = x, x is a real number

Answer

(i) f(x) = 2 –3x, x ∈ R, x > 0

Class XI Chapter 2 –Relations and Functions Maths

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The values of f(x) for various values of real numbers x > 0 can be written in the tabular

form as

x 0.01 0.1 0.9 1 2 2.5 4 5 …

f(x) 1.97 1.7 –.7 – – –.5 –0 –3 …

Thus, it can be clearly observed that the range of f is the set of all real numbers less

than 2.

i.e., range of f = (–, 2)

Alter:

Let x > 0

⇒ 3x > 0

⇒ 2 –x < 2

⇒ f(x) < 2

∴Range of f = (–, 2)

(ii) f(x) = x2 + 2, x, is a real number

The values of f(x) for various values of real numbers x can be written in the tabular form

as

x 0 }0.3 }0.8 }1 }2 }3 …

f(x) 2 2.09 2.64 3 6 11 …..

Thus, it can be clearly observed that the range of f is the set of all real numbers greater

than 2.

i.e., range of f = [2, )

Alter:

Let x be any real number.

Accordingly,

x2 ≥0

⇒ x2 + 2 ≥0 + 2

⇒ x2 + 2 ≥2

⇒ f(x) ≥2

∴ Range of f = [2, )

(iii) f(x) = x, x is a real number

It is clear that the range of f is the set of all real numbers.

∴ Range of f = R

Text solution

Class XI Chapter 2 –Relations and Functions Maths

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Question 1:

The relation f is defined by

The relation g is defined by

Show that f is a function and g is not a function.

Answer

The relation f is defined as

It is observed that for

0 ≤x < 3, f(x) = x2

3 < x ≤10, f(x) = 3x

Also, at x = 3, f(x) = 32 = 9 or f(x) = 3 ~ 3 = 9

i.e., at x = 3, f(x) = 9

Therefore, for 0 ≤x ≤10, the images of f(x) are unique.

Thus, the given relation is a function.

The relation g is defined as

It can be observed that for x = 2, g(x) = 22 = 4 and g(x) = 3 ~ 2 = 6

Hence, element 2 of the domain of the relation g corresponds to two different images

i.e., 4 and 6. Hence, this relation is not a function.

Question 2:

If f(x) = x2, find .

Answer

Question 3:

Find the domain of the function

Answer

The given function is .

Class XI Chapter 2 –Relations and Functions Maths

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It can be seen that function f is defined for all real numbers except at x = 6 and x = 2.

Hence, the domain of f is R –{2, 6}.

Question 4:

Find the domain and the range of the real function f defined by .

Answer

The given real function is .

It can be seen that is defined for (x –1) ≥0.

i.e., is defined for x ≥1.

Therefore, the domain of f is the set of all real numbers greater than or equal to 1 i.e.,

the domain of f = [1, ).

As x ≥1 ⇒ (x –1) ≥0 ⇒

Therefore, the range of f is the set of all real numbers greater than or equal to 0 i.e., the

range of f = [0, ).

Question 5:

Find the domain and the range of the real function f defined by f (x) = |x –1|.

Answer

The given real function is f (x) = |x –1|.

It is clear that |x –1| is defined for all real numbers.

∴Domain of f = R

Also, for x ∈ R, |x –1| assumes all real numbers.

Hence, the range of f is the set of all non-negative real numbers.

Question 6:

Let be a function from R into R. Determine the range of f.

Answer

The range of f is the set of all second elements. It can be observed that all these

elements are greater than or equal to 0 but less than 1.

[Denominator is greater numerator]

Thus, range of f = [0, 1)

Question 7:

Class XI Chapter 2 –Relations and Functions Maths

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Let f, g: R → R be defined, respectively by f(x) = x + 1, g(x) = 2x –3. Find f + g, f –g

and .

Answer

f, g: R → R is defined as f(x) = x + 1, g(x) = 2x –3

(f + g) (x) = f(x) + g(x) = (x + 1) + (2x –3) = 3x –2

∴(f + g) (x) = 3x –2

(f –g) (x) = f(x) –g(x) = (x + 1) –(2x –3) = x + 1 –2x + 3 = –x + 4

∴ (f –g) (x) = –x + 4

Question 8:

Let f = {(1, 1), (2, 3), (0, –), (–, –)} be a function from Z to Z defined by f(x) = ax

+ b, for some integers a, b. Determine a, b.

Answer

f = {(1, 1), (2, 3), (0, –), (–, –)}

f(x) = ax + b

(1, 1) ∈ f

⇒ f(1) = 1

⇒ a ~ 1 + b = 1

⇒ a + b = 1

(0, –) ∈ f

⇒ f(0) = –

⇒ a ~ 0 + b = –

⇒ b = –

On substituting b = – in a + b = 1, we obtain a + (–) = 1 ⇒ a = 1 + 1 = 2.

Thus, the respective values of a and b are 2 and –.

Question 9:

Let R be a relation from N to N defined by R = {(a, b): a, b ∈ N and a = b2}. Are the

following true?

(i) (a, a) ∈ R, for all a ∈ N (ii) (a, b) ∈ R, implies (b, a) ∈ R

(iii) (a, b) ∈ R, (b, c) ∈ R implies (a, c) ∈ R.

Justify your answer in each case.

Answer

R = {(a, b): a, b ∈ N and a = b2}

(i) It can be seen that 2 ∈ N;however, 2 ≠ 22 = 4.

Therefore, the statement “(a, a) ∈ R, for all a ∈ N” is not true.

(ii) It can be seen that (9, 3) ∈ N because 9, 3 ∈ N and 9 = 32.

Now, 3 ≠ 92 = 81; therefore, (3, 9) ∉N

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Therefore, the statement “(a, b) ∈ R, implies (b, a) ∈ R” is not true.

(iii) It can be seen that (9, 3) ∈ R, (16, 4) ∈ R because 9, 3, 16, 4 ∈ N and 9 = 32 and 16

= 42.

Now, 9 ≠ 42 = 16; therefore, (9, 4) ∉N

Therefore, the statement “(a, b) ∈ R, (b, c) ∈ R implies (a, c) ∈ R” is not true.

Question 10:

Let A = {1, 2, 3, 4}, B = {1, 5, 9, 11, 15, 16} and f = {(1, 5), (2, 9), (3, 1), (4, 5), (2,

11)}. Are the following true?

(i) f is a relation from A to B (ii) f is a function from A to B.

Justify your answer in each case.

Answer

A = {1, 2, 3, 4} and B = {1, 5, 9, 11, 15, 16}

∴A × B = {(1, 1), (1, 5), (1, 9), (1, 11), (1, 15), (1, 16), (2, 1), (2, 5), (2, 9), (2, 11),

(2, 15), (2, 16), (3, 1), (3, 5), (3, 9), (3, 11), (3, 15), (3, 16), (4, 1), (4, 5), (4, 9), (4,

11), (4, 15), (4, 16)}

It is given that f = {(1, 5), (2, 9), (3, 1), (4, 5), (2, 11)}

(i) A relation from a non-empty set A to a non-empty set B is a subset of the Cartesian

product A ~ B.

It is observed that f is a subset of A ~ B.

Thus, f is a relation from A to B.

(ii) Since the same first element i.e., 2 corresponds to two different images i.e., 9 and

11, relation f is not a function.

Question 11:

Let f be the subset of Z ~ Z defined by f = {(ab, a + b): a, b ∈ Z}. Is f a function from Z

to Z: justify your answer.

Answer

The relation f is defined as f = {(ab, a + b): a, b ∈ Z}

We know that a relation f from a set A to a set B is said to be a function if every element

of set A has unique images in set B.

Since 2, 6, –, – ∈ Z, (2 ~ 6, 2 + 6), (– × –, – + (–)) ∈ f

i.e., (12, 8), (12, –) ∈ f

It can be seen that the same first element i.e., 12 corresponds to two different images

i.e., 8 and –. Thus, relation f is not a function.

Question 12:

Let A = {9, 10, 11, 12, 13} and let f: A → N be defined by f(n) = the highest prime

factor of n. Find the range of f.

Answer

A = {9, 10, 11, 12, 13}

f: A → N is defined as

f(n) = The highest prime factor of n

Prime factor of 9 = 3

Prime factors of 10 = 2, 5

Prime factor of 11 = 11

Prime factors of 12 = 2, 3

Prime factor of 13 = 13

∴f(9) = The highest prime factor of 9 = 3

Class XI Chapter 2 –Relations and Functions Maths

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f(10) = The highest prime factor of 10 = 5

f(11) = The highest prime factor of 11 = 11

f(12) = The highest prime factor of 12 = 3

f(13) = The highest prime factor of 13 = 13

The range of f is the set of all f(n), where n ∈ A.

∴Range of f = {3, 5, 11, 13}