## How to derive Einstein's radius equation?

I have been trying to show that the Einstein ring radius R_e = 2[GMx(L-x)/(Lc^2)]^1/2
to no avail. can someone who knows this show me, or at least point out the direction. I have a strong hunch that i'll have to use the light bending formula for small angles 
delta_phi = 4GM/(bc^2), and geometry.

The Einstein radius is the radius of the ring image formed when a bright source is exactly behind a spherically symmetric lens mass. L is the distance to the source, and x is the distance to the lens mass.