math
posted by Chris .
constructanglefsothatmanglefequals2manglec

Given angle c, ACB
construct a circle with center C intersecting AC at D and CB at E.
Using D as a center, construct a circle with radius DE so it intersects the circle C at F.
Angle FCA = ACB since they subtend equal arcs.
So, FCB = FCA+ACB = 2*ACB