Problem

Source: 2010 Oral Moscow Geometry Olympiad grades 10-11 p2

Tags: geometry, cyclic quadrilateral, perpendicular, parallel



Quadrangle ABCD is inscribed in a circle. The perpendicular from the vertex C on the bisector of ABD intersects the line AB at the point C1. The perpendicular from the vertex B on the bisector of ACD intersects the line CD at the point B1. Prove that B1C1AD.