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RU-ALLRUSSIAN 2009 Problem 107

  • Grade 10

The incircle (I)(I) of a given scalene triangle ABCABC touches its sides BCBC , CACA , ABAB at A1A_1 , B1B_1 , C1C_1 , respectively. Denote ωB\omega_B , ωC\omega_C the incircles of quadrilaterals BA1IC1BA_1IC_1 and CA1IB1CA_1IB_1 , respectively. Prove that the internal common tangent of ωB\omega_B and ωC\omega_C different from IA1IA_1 passes through AA .

Dataset text AI-MO AoPS. Quoted with source.

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