NCERT Solutions
Class 12 Maths
Relations and Functions

Ex. 1.4 Q9
Let * be a binary operation on the set Q of rational numbers as follows:
(i) a * b = a − b (ii) a * b = a2 + b2 (iii) a * b = a + ab
(iv) a * b = (a − b)2 (v) a ∗ b = ab/4 (vi) a * b = ab2
Find which of the binary operations are commutative and which are associative.
(i) On Q, the operation * is defined as a * b = a − b. It can be observed that:
(1/2) * (1/3) = 1/2 - 1/3 = (3 - 2)/6 = 1/6 and
(1/3) * (1/2) = 1/3 - 1/2 = (2 - 3)/6 = -1/6
So, (1/2) * (1/3) ≠ (1/3) * (1/2), where 1/2, 1/3 ∈ Q
Thus, the operation * is not commutative.
It can also be observed that
(1/2 * 1/3) * 1/4 = (1/2 - 1/3) * 1/4 = (3 - 2)/6 * 1/4 = 1/6 * 1/4 = (2 - 3)/12 = -1/12 and
1/2 *( 1/3 * 1/4) = 1/2 * (1/3 - 1/4) = 1/2 * (4 - 3)/12 = 1/2 * 1/12 = (6 - 1)/12 = 5/12
So, (1/2 * 1/3) * 1/4 ≠ 1/2 * (1/3 * 1/4), where 1/2, 1/3, 1/4 ∈ Q
Thus, the operation * is not commutative.
(ii) On Q, the operation * is defined as a * b = a2 + b2
For a, b ∈ Q, we have
a * b = a2 + b2 = b2 + a2 = b * a
So, a * b = b * a
Thus, the operation * is commutative.
It can be observed that
(1 * 2) * 3 = (12 + 22) * 3 = (1 + 4) * 3 = 5 * 3 = 52 + 32 = 34 and
1 * (2 * 3) = 1 * (22 + 32) = 1 * (4 + 9) = 1 * 13 = 12 + 132 = 170
So, (1 * 2) * 3 ≠ 1 * (2 * 3), where 1, 2, 3 ∈ Q
Thus, the operation * is not associative.
(iii) On Q, the operation * is defined as a * b = a + ab.
It can be observed that
1 * 2 = 1 + 1 × 2 = 1 + 2 = 3
2 * 1 = 2 + 2 × 1 = 2 + 2 = 4
So, 1 * 2 ≠ 2 * 1, where 1, 2 ∈ Q
Thus, the operation * is not commutative.
It can also be observed that
(1 * 2) * 3 = (1+ 1 × 2) * 3 = (1 + 2) * 3 = 3 * 3 = 3 + 3 × 3 = 3 + 9 = 12 and
1 * (2 * 3) = 1 * (2 + 2 × 3) = 1 * (2 + 6) = 1 * 8 = 1 + 1 × 8 =1 + 8 = 9
So, (1 * 2) * 3 ≠ 1 * (2 * 3), where 1, 2, 3 ∈ Q
Thus, the operation * is not associative.
(iv) On Q, the operation * is defined by a * b = (a − b)2
For a, b ∈ Q, we have
a * b = (a − b)2
b * a = (b − a)2 = [− (a − b)]2 = (a − b)2
So, a * b = b * a
Thus, the operation * is commutative.
It can be observed that
(1 * 2) * 3 = (1 – 2)2 * 3 = (– 1)2 * 3 = 1 * 3 = (1 – 3)2 = (– 2)2 = 4
and
1 * (2 * 3) = 1 * (2 – 3)2 = 1 * (– 1)2 = 1 * 1 = (1 – 1)2 = 0
So, (1 * 2) * 3 ≠ 1 * (2 * 3), where 1, 2, 3 ∈ Q
Thus, the operation * is not associative.
(v) On Q, the operation * is defined as a*b = ab/4
For a, b ∈ Q, we have
a*b = ab/4 = ba/4 = b*a
So, a * b = b * a
Thus, the operation * is commutative.
For a, b, c ∈ Q, we have
(a ∗ b) ∗ c = (ab/4) ∗ c = {(ab/4) . c}/4 = abc/16 and
a ∗ (b ∗ c) = a ∗ ( bc/4) = {a. (bc/4)}/4 = abc/16
So, (a * b) * c = a * (b * c), where a, b, c ∈ Q
Thus, the operation * is associative.
(vi) On Q, the operation * is defined as a * b = ab2
It can be observed that
(1/2) * (1/3) = (1/2) * (1/3)2 = (1/2).(1/9) = 1/18 and
(1/3) * (1/2) = (1/3) * (1/2)2 = (1/3).(1/4) = 1/12
So, (1/2) * (1/3) ≠ (1/3) * (1/2), where 1/2 and 1/3 ∈ Q
Thus, the operation * is not commutative.
It can also be observed that
(1/2 * 1/3) * 1/4 = [1/2(1/3)2] ∗ 1/4 = 1/18 ∗ 1/4 = 1/18 . (1/4)2 = 1/(18 × 16) = 1/288 and
1/2 * (1/3 * 1/4) = (1/2)[1/3(1/4)2] = 1/2 ∗ 1/48 = 1/2 . (1/48)2 = 1/(2 × 2304) = 1/4608
So, (1/2 * 1/3) * 1/4 ≠ 1/2 * (1/3 * 1/4) , where 1/2, 1/3, 1/4 ∈ Q
Thus, the operation * is not associative.
Hence, the operations defined in (ii), (iv), (v) are commutative and the operation defined in (v)
is associative.