olymmath

JBMO-SL 2016 G1

Let ABCA B C be an acute angled triangle, let OO be its circumcentre, and let D,E,FD, E, F be points on the sides BC,AC,ABB C, A C, A B, respectively. The circle (c1)\left(c_{1}\right) of radius FAF A, centred at FF, crosses the segment (OA)(O A) at A′A^{\prime} and the circumcircle (c) of the triangle ABCA B C again at KK. Similarly, the circle (c2)\left(c_{2}\right) of radius DBD B, centred at DD, crosses the segment (OB)(O B) at B′B^{\prime} and the circle (c) again at LL. Finally, the circle (c3)\left(c_{3}\right) of radius ECE C, centred at EE, crosses the segment (OC)(O C) at C′C^{\prime} and the circle (c) again at MM. Prove that the quadrilaterals BKFA′B K F A^{\prime}, CLDB′C L D B^{\prime} and AMEC′A M E C^{\prime} are all cyclic, and their circumcircles share a common point.

Dataset text NuminaMath olympiads. Quoted with source.

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