uwzx forms a trapazoid with sides of length a,a+c,c,2*sqr(ac) per the pythagorean formula. Likewise the bottom of trapazoid vyzw is 2*sqr(bc) and the bottom of trapazoid uvyx is 2*sqr(ab).

The sum of the bottoms of trapazoids uwzx and vyzw is equal to the bottom of trapazoid uvyx:

2*sqr(ac)+2*sqr(bc)=2*sqr(ab)

sqr(ac)+sqr(bc)=sqr(ab)

sqr(c)*(sqr(a)+sqr(b))=sqr(ab)

sqr(c)=sqr(ab)/(sqr(a)+sqr(b))

c=ab/(a+2*sqr(ab)+b)

**Note**: Steve Schaefer points out the following general relationship c^{-1/2} = a^{-1/2} + b^{-1/2}