For each binary operation * defined below, determine whether * is commutative or associative.
(i) On Z, define a * b=a−b
(ii) On Q, define a * b=ab+1
(iii) On Q, define a * b= \(\frac {ab}{2}\).
(iv) On Z+, define a * b=2ab
(v) On Z+, define a * b=ab
(vi) On R−{−1},define a * b= \(\frac {a}{b+1}\)