Let Q+ be the set of all positive rational numbers.
(i) Show that the operation * on Q+ defined by  is a binary operation.
 is a binary operation.
(ii) Show that * is commutative.
(iii) Show that * is not associative.
(i) * is an operation as  where a, b ∈ Q+. Let
 where a, b ∈ Q+. Let  and b = 2 two integers.
 and b = 2 two integers.

So, * is a binary operation from  .
.
(ii) For commutative binary operation, a*b = b*a.

Since a*b = b*a, hence * is a commutative binary operation.
(iii) For associative binary operation, a*(b*c) = (a*b) *c.


As a*(b*c) ≠(a*b) *c, hence * is not associative binary operation.