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ASU 213 All Soviet Union MO 1975 flies along triangle perimeter, centroid

Source:

July 5, 2019
geometryperimeterCentroid

Problem Statement

Three flies are crawling along the perimeter of the triangle ABCABC in such a way, that the centre of their masses is a constant point. One of the flies has already passed along all the perimeter. Prove that the centre of the flies' masses coincides with the centre of masses of the triangle ABCABC . (The centre of masses for the triangle is the point of medians intersection.