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Baltic Way 2019

Baltic Way 2019 geometry

Problem

Let be a triangle and its orthocenter. Point lies on segment and is its foot onto the line . Prove that iff bisects .
Solution
Let . Then , so is orthocenter is parallelogram bisects .

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleAngle chasing