What is the claw theorem?

The so-called projection is the orthographic projection.

Among them, the vertical foot from a point to a straight line is called the orthogonal projection of the point on this straight line. The line segment between the orthographic projections of two endpoints of a line segment on a straight line is called the orthographic projection of the line segment on the straight line.

From the similar nature of triangles:

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-angled edge is the median of the projection of this right-angled edge on the hypotenuse and the proportion of the hypotenuse.

In an arbitrary triangle ABC

a=b*cosC+c*cosB

b=c*cosA+a*cosC

c=a*cosB+b*cosA

A plane A, a line L, L is a diagonal of this plane, that is, it intersects this plane, and the intersection point is O, but it is not vertical. If it intersects with L, it is the vertical plane A of L', its vertical foot is A, and the intersection point connecting AO, L and L' is B. If a straight line M belongs to A and is perpendicular to L, it must be perpendicular to L'

This is the projective theorem

As for the melon seed theorem, I may feel like melon seeds.

The spiral law of the right hand upstairs seems to be physical ... sweating ~