Let , , be points on the sides , , respectively of a triangle . Lines , , intersect the circumcircle of again at , , , respectively.Show that: and find the cases of equality.
KMO 1998
Korean Mathematical Olympiad 1998.
Other problems
Let be the incenter of triangle , a circle through tangent to , and a circle through tangent to . Prove that , and the circumcircle of have a common point.