High Quality Content by WIKIPEDIA articles! Menelaus' theorem, attributed to Menelaus of Alexandria, is a theorem about triangles in plane geometry.This is one of many proofs for this theorem. The sign of the left-hand side of the theorem's equation can be checked. Line DEF must intersect the sides of triangle ABC an even number of times—either twice altogether, if it passes into the triangle and out again (upper diagram), or not at all, if it misses the triangle (lower diagram) (Pasch's axiom). Hence there are an odd number of negative terms, and the total product is negative.

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