You can't prove that B is the midpoint of AC. Consider the following situation:
The measure of AC is 4 units and the measure of AB is 2 units. 2 times 2 is certainly 4, but B is certainly not the midpoint of AC.
Now if you had bothered to mention that B was located on the line segment AC, that would have been a different story. But that isn't what you asked, is it?
Extra credit: Why would it have been insufficient to state that A, B, and C are collinear?
John
My calculator said it, I believe it, that settles it
