Menelaus, an ancient Greek mathematician, first proved Menelaus theorem. It points out that if a straight line intersects with three sides AB, BC, CA of △ABC or its extension lines at points F, D and E, then (AF/FB )× (BD/DC )× (CE/EA) =1. Or: let x, y and z be on the straight lines of BC, CA and AB of △ABC, then the necessary and sufficient conditions of x, y and Z*** lines are (az/zb) * (bx/xc) * (cy/ya) =1.