olymmath

ISL 2018 G7

    Let OO be the circumcentre, and Ω\Omega be the circumcircle of an acute-angled triangle ABCA B C. Let PP be an arbitrary point on Ω\Omega, distinct from A,B,CA, B, C, and their antipodes in Ω\Omega. Denote the circumcentres of the triangles AOP,BOPA O P, B O P, and COPC O P by OA,OBO_{A}, O_{B}, and OCO_{C}, respectively. The lines ℓA,ℓB\ell_{A}, \ell_{B}, and ℓC\ell_{C} perpendicular to BC,CAB C, C A, and ABA B pass through OA,OBO_{A}, O_{B}, and OCO_{C}, respectively. Prove that the circumcircle of the triangle formed by ℓA,ℓB\ell_{A}, \ell_{B}, and ℓC\ell_{C} is tangent to the line OPO P. (Russia)

    Dataset text NuminaMath olympiads. Quoted with source.

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