olymmath

BALKAN-SL 2018 G5

    Let ABCA B C be an acute-angled triangle with AB<AC<BCA B<A C<B C and let DD be an arbitrary point on the extension of BCB C beyond CC. The circle γ(A,AD)\gamma(A, A D) intersects the rays ACA C, AB,CBA B, C B at points E,F,GE, F, G, respectively. The circumcircle ω1\omega_{1} of triangle AFGA F G intersects the lines FE,BC,GE,DFF E, B C, G E, D F again at points J,H,H′,J′J, H, H^{\prime}, J^{\prime}. The circumcircle ω2\omega_{2} of triangle ADEA D E intersects the lines FE,BC,GE,DFF E, B C, G E, D F again at points I,K,K′,I′I, K, K^{\prime}, I^{\prime}. Prove that the quadrilaterals HIJKH I J K and H′I′J′K′H^{\prime} I^{\prime} J^{\prime} K^{\prime} are cyclic and that their circumcenters coincide. (Greece)

    Dataset text NuminaMath olympiads. Quoted with source.

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