How does a*b relates to a^2 and b^2 ?

Question

What is the shortest distance between circle A defined by (x − 5)2 + (y − 4)2 = 4 and circle B defined by (x − 1)2 + (y − 1)2 = 1? There is some point P on circle A closest to B, and there is another point Q on circle B closest to A. What are the coordinates of P and Q?

Answer 1

Here it is:
a*b=[(a+b)^2-(a^2+b^2)]/2