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Orthocenters form an equilateral triangle

Source: 2019-20 International Dürer Competition, Category E ,P4

August 19, 2020
geometryEquilateral Triangle

Problem Statement

Let ABCABC be an acute triangle with side ABAB of length 11. Say we reflect the points AA and BB across the midpoints of BCBC and ACAC, respectively to obtain the points AA’ and BB’ . Assume that the orthocenters of triangles ABC ABC, ABCA’BC and BACB’AC form an equilateral triangle. a) Prove that triangle ABCABC is isosceles. b) What is the length of the altitude of ABCABC through CC?