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RS-SMO 2011 Problem 3

    Let HH be orthocenter and OO circumcenter of an acuted angled triangle ABCABC . DD and EE are feets of perpendiculars from AA and BB on BCBC and ACAC respectively. Let ODOD and OEOE intersect BEBE and ADAD in KK and LL , respectively. Let XX be intersection of circumcircles of HKDHKD and HLEHLE different than HH , and MM is midpoint of ABAB . Prove that K,L,MK, L, M are collinear iff XX is circumcenter of EODEOD .

    Dataset text AI-MO AoPS. Quoted with source.

    Official text: Serbian Mathematical Olympiad 2011 official paper, page 1.

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