Let L, M, and N be points on the sides BC, CA, and AB of a triangle (possibly extended). Then Menelaus’ Theorem states that L, M, and N are collinear if and only if

(Note that BC, for example, is considered to be directed from B to C so that LC, for example, is positive if LC is in the same direction as BC and negative if it is in the opposite direction.) Compare Ceva’s Theorem.