Projection theorem: in Rt△abC, ∠ ACB = 90, and cd is the height on the hypotenuse AB, then there are: ① CD 2; = AD DB, ② BC 2 = BD Ba, ③ AC 2 = AD AB; ④ AC BC = AB CD (equal product formula, which can be proved by area).

Menelaus theorem: 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.