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RMM 2020 Problem 1

  • Day 1

Let ABCABC be a triangle with a right angle at CC . Let II be the incentre of triangle ABCABC , and let DD be the foot of the altitude from CC to ABAB . The incircle ω\omega of triangle ABCABC is tangent to sides BCBC , CACA , and ABAB at A1A_1 , B1B_1 , and C1C_1 , respectively. Let EE and FF be the reflections of CC in lines C1A1C_1A_1 and C1B1C_1B_1 , respectively. Let KK and LL be the reflections of DD in lines C1A1C_1A_1 and C1B1C_1B_1 , respectively.

Prove that the circumcircles of triangles A1EIA_1EI , B1FIB_1FI , and C1KLC_1KL have a common point.

Dataset text AI-MO AoPS. Quoted with source.

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