What's the use of projective theorem?

Projection Theorem, also known as Euclid Theorem: In a right triangle, the height on the hypotenuse is the median term of the ratio of the projection of two right-angled sides on the hypotenuse, and each right-angled side is the median term of the ratio of the projection of this right-angled side on the hypotenuse to the hypotenuse. It is an important theorem of mathematical graphic calculation.

In the overview, at Rt△ABC, ∠ ABC = 90 and BD is the height on the hypotenuse AC, then the projective theorem is as follows:

BD？ = Advertising CD

AB？ = exchange advertising

BC? =CD AC

It was put forward by Euclid, a famous mathematician in ancient Greece and the author of The Elements of Geometry.