olymmath

OSN 2020 Problem 1

    Since this is already 3 PM (GMT +7) in Jakarta, might as well post the problem here.

    Problem 1. Given an acute triangle ABCABC and the point DD on segment BCBC . Circle c1c_1 passes through A,DA, D and its centre lies on ACAC . Whereas circle c2c_2 passes through A,DA, D and its centre lies on ABAB . Let P≠AP \neq A be the intersection of c1c_1 with ABAB and Q≠AQ \neq A be the intersection of c2c_2 with ACAC . Prove that ADAD bisects ∠PDQ\angle{PDQ} .

    Dataset text AI-MO AoPS. Quoted with source.

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