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Brazilian Math Olympiad

Brazil geometry

Problem

Let be an acute triangle and is orthocenter. Let be the intersection of and and be the intersection of and . The circumcircle of meets the circumcircle of at . Prove that the angle bisectors of and concur at a point on line .

problem
Solution


By the angle bisector theorem, it suffices to prove that . We have and , so triangles and are similar. Thus

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTriangle trigonometryCyclic quadrilateralsCirclesConcurrency and CollinearityAngle chasing