Question

a*b = a+b-5 on R, Examine whether the given binary operations are commutative or not and associative or not.

Answer 1

In the attachment I have answered this problem.     Concept:    * is commutative if a * b = b * a for all a.    * is associative if a*(b*c) = (a*b)*c for a,b,c     See the attachment for detailed solution