Geometry · Squares · Equilateral triangles · Congruent triangles · Angle chasing
A square \(ABCD\) is given, together with points \(E\) and \(F\) lying outside the square such that the triangles \(BEC\) and \(CFD\) are equilateral.
Prove that the triangle \(AEF\) is equilateral as well.
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Slovenian High School Mathematics Competition for Vega Awards (MaSSA), drzavno (national) round 2015, 1. letnik, category A, problem 3. Organized by DMFA Slovenije (Society of Mathematicians, Physicists and Astronomers of Slovenia). Source