Let (P,r) denote a circle with center P and radius r.

Circles (A,a) and (B,b) are externally tangent with 
a less than b.

Let CD and EF be the common external tangents to
circles (A,a) and (B,b) with C and F on circle (A,a) 
and D and E on circle (B,b).

What is the area of trapezoid CDEF in terms of 
a and b?