I need help setting up this problem.

Let Q be the set of positive rationals. Consider the operation * defined by a*b =ab/a+b. Prove that * is an associative, commutative binary operation on Q. I need to make sure i clearly identify and state places in the proof where you use properties of the operations + and . on Q.

I know what I need to do, I just do not know how to set up the proof. If I need to list what I have then I can. It would be great to get some feed back.