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Four coplanar points

Source: Canadian Mathematical Olympiad 1976 Problem 6

October 23, 2008
geometry proposedgeometry

Problem Statement

If A,B,C,D A,B,C,D are four points in space, such that \angle ABC\equal{}\angle BCD\equal{}\angle CDA\equal{}\angle DAB\equal{}\pi/2, prove that A,B,C,D A,B,C,D lie in a plane.
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