RS-SMO 2011 Problem 3
Let be orthocenter and circumcenter of an acuted angled triangle . and are feets of perpendiculars from and on and respectively. Let and intersect and in and , respectively. Let be intersection of circumcircles of and different than , and is midpoint of . Prove that are collinear iff is circumcenter of .
Dataset text AI-MO AoPS. Quoted with source.
Official text: Serbian Mathematical Olympiad 2011 official paper, page 1.
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