34th United States of America Mathematical Olympiad-P3

Let ABC be an acute-angled triangle, and let P and Q be two points on side BC. Construct point C1 in such a way that convex quadrilateral APBC1 is cyclic, QC1 CA, and C1 and Q lie on opposite sides of line AB. Construct point B1 in such a way that convex quadrilateral APCB1 is cyclic, QB1 BA, and B1 and Q lie on opposite sides of line AC. Prove that points B1,C1,P, and Q lie on a circle.

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