Geometry · Perpendicular bisectors · Concyclic points · Simson line
Three distinct points \(A\), \(B\), \(C\) lie on a line \(\ell\), and a point \(O\) lies off \(\ell\). The perpendicular bisectors of the segments \(OA\), \(OB\) and \(OC\) form a triangle \(EFG\). Prove that the points \(O\), \(E\), \(F\) and \(G\) lie on one circle.
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Serbian Municipal Competition 2020, high school grade I, category A, problem 1. Organized by the Mathematical Society of Serbia (DMS). Source