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ELMO 2021 Problem 1

    In △ABC\triangle ABC , points PP and QQ lie on sides ABAB and ACAC , respectively, such that the circumcircle of △APQ\triangle APQ is tangent to BCBC at DD . Let EE lie on side BCBC such that BD=ECBD = EC . Line DPDP intersects the circumcircle of △CDQ\triangle CDQ again at XX , and line DQDQ intersects the circumcircle of △BDP\triangle BDP again at YY . Prove that DD , EE , XX , and YY are concyclic.

    Dataset text AI-MO AoPS. Quoted with source.

    Official text: ELMO 2021 official paper, page 1.

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