Check out Terry Lee's book - he's got a few proofs.
An easy way is to do the sin of the angles you're trying to get equal. Then use perpendicular distances from the foci to the tangent and it comes out easily from there.
2000 Q7b is a question nice question on the reflection property, although reflection wasn't mentioned per se.
It is a nice question because this is how physicists look at reflection in Optics.