Geometry · Square · Congruent triangles · Collinearity

Problem 1, 2012

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CityProof

Let \(ABCD\) be a square and let \(E\) be the midpoint of its side \(CD\). The line through \(D\) perpendicular to the diagonal \(BD\) meets the line \(AE\) at a point \(F\). Prove that the points \(B\), \(C\) and \(F\) are collinear.

A B C D E F
The perpendicular to \(BD\) at \(D\) and the line \(AE\) meet at \(F\).

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Serbian Municipal Competition 2012, high school grade I, category A, problem 1. Organized by the Mathematical Society of Serbia (DMS). Source