The projection theorem of a right triangle, also known as Euclid's theorem, is that the height of the hypotenuse in a right triangle is the proportional average of the projections of two right angles on the hypotenuse, and each right angle is the proportional average of the projection of this right angle on the hypotenuse and the hypotenuse. The formula is as follows: in Rt△abC ∠ ACB = 90, and cd is the height on the hypotenuse AB, then the projective theorem is as follows: ①CD? =AD DB，②BC？ =BD BA，③AC？ = AD AB④AC BC=AB CD