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segments of midpoints of incenters and excenters perpendicular to side

Source: 2018 Oral Moscow Geometry Olympiad grades 10-11 p4

July 25, 2019
geometrymidpointincenterexcenter

Problem Statement

On the side ABAB of the triangle ABCABC, point MM is selected. In triangle ACMACM point I1I_1 is the center of the inscribed circle, J1J_1 is the center of excircle wrt side CMCM. In the triangle BCMBCM point I2I_2 is the center of the inscribed circle, J2J_2 is the center of excircle wrt side CMCM. Prove that the line passing through the midpoints of the segments I1I2I_1I_2 and J1J2J_1J_2 is perpendicular to ABAB.