Let BD be the bisector of angle B in triangle ABC. The circumcircle of triangle BDC meets AB at E, while the circumcircle of triangle ABD meets BC at F. Prove that AE = CF.
By power-of-a-point, AE · AB = AD · AC and CF · CB = CD · CA, so AE/CF = (AD/CD)(BC/AB). However, AB/CB = AD/CD by the angle bisector theorem, so AE = CF.