IMO Shortlist 2007 problem G8


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Point P lies on side AB of a convex quadrilateral ABCD. Let \omega be the incircle of triangle CPD, and let I be its incenter. Suppose that \omega is tangent to the incircles of triangles APD and BPC at points K and L, respectively. Let lines AC and BD meet at E, and let lines AK and BL meet at F. Prove that points E, I, and F are collinear.

Author: Waldemar Pompe, Poland
Izvor: Međunarodna matematička olimpijada, shortlist 2007