NCERT Solutions
Class 12 Maths
Relations and Functions

Ex. 1.4 Q11
Let 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.
Given, 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) ∈ A
Then, a, b, c, d ∈ 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]
So, (a, b) * (c, d) = (c, d) * (a, b)
Therefore, the operation * is commutative.
Now, let (a, b), (c, d), (e, f) ∈ A
Then, a, b, c, d, e, f ∈ 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)
So, [(a, b) ∗ (c, d)] ∗ (e, f) = (a, b) ∗ [(c, d) ∗ (e, f)]
Therefore, the operation * is associative.
Let an element e = (e1, e2) ∈ A will be an identity element for the operation *
if a * e = a = e * a for all a = (a1, a2) ∈ A
i.e., (a1 + e1, a2 + e2) = (a1, a2) = (e1 + a1, e2 + a2)
which is not true for any element in A.
Therefore, the operation * does not have any identity element.