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angle wanted, centroid of AMN=orthocenter of ABC (Kyiv City Olympiad 2004 10.5 )

Source:

June 27, 2021
geometryCentroidorthocenter

Problem Statement

Let the points MM and NN in the triangle ABCABC be the midpoints of the sides BCBC and ACAC, respectively. It is known that the point of intersection of the altitudes of the triangle ABCABC coincides with the point of intersection of the medians of the triangle AMNAMN. Find the value of the angle ABCABC.