What is the formula of projective theorem?

Projection theorem of right triangle (also called Euclid theorem): In a right triangle, the height on the hypotenuse is the proportional average of the projections of two right angles on the hypotenuse. Each right angle is the projection of this right angle on the hypotenuse and the proportional average of the hypotenuse.

In the formula Rt△ABC, ∠ BAC = 90, and AD is the height on the hypotenuse BC, then the projective theorem is as follows: (1) (ad) 2; = BD dc,(2)(ab)^2； =BD BC，

(3)(ac)^2； = CD BC。 Equal product formula (4)ABXAC=BCXAD (available area proof) The projective theorem of any triangle is also called the "first cosine theorem": the three sides of △ABC are A, B and C respectively, and the angles they face are A, B and C respectively, so there are A = b cosC+c cosB and B = C.