State whether the following statements are true or false. Justify.

(i) For an arbitrary binary operation * on a set N, a * a = a a * N.

(ii) If * is a commutative binary operation on N, then a * (b * c) = (c * b) * a

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Solution

(i) Define an operation * on N as:

a * b = a + b a, b ∈ N

Then, in particular, for b = a = 3, we have:

3 * 3 = 3 + 3 = 6 ≠ 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.

∴ a * (b * c) = (c * b) * a

Therefore, statement (ii) is true.

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