The following theorem, due to Menelaus of Alexandria:
Let L, M and N be points on the sides BC, CA and AB of a triangle (possibly extended). Then 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; see measure.) See Ceva's Theorem.