OBM 2021 Problem 1
Sejam um quadrilátero convexo no plano e , , e os circuncentros dos triângulos , , e , respectivamente. Suponha que esses quatro circuncentros sejam pontos distintos. Prove que esses pontos não estão em uma mesma circunferência.
Official text Brazilian Mathematical Olympiad 2021 official paper, page 1. Quoted with source.
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