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On sides AB and BC of a square ABCD respective points E and F

Source: 1979 Austrian-Polish Mathematical Competition p1

February 15, 2020
geometrysquareright angleequal segments

Problem Statement

On sides ABAB and BCBC of a square ABCDABCD the respective points EE and FF have been chosen so that BE=BFBE = BF. Let BNBN be the altitude in triangle BCEBCE. Prove that ∠DNF=90\angle DNF = 90.
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