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incenter lies on perpendicular, other incircle related

Source: 2009 Oral Moscow Geometry Olympiad grades 10-11 p3

September 14, 2020
geometryincenterperpendicularincircle

Problem Statement

Altitudes AA1AA_1 and BB1BB_1 are drawn in the acute-angled triangle ABCABC. Prove that the perpendicular drawn from the touchpoint of the inscribed circle with the side BCBC, on the line ACAC passes through the center of the inscribed circle of the triangle A1CB1A_1CB_1.
(V. Protasov)