Pythagorean Theorem: It is a basic geometric theorem, which means that the sum of squares of two right angles of a right triangle is equal to the square of the hypotenuse. In ancient China, right-angled triangles were called pythagorean, and the smaller right-angled side was a hook, the other long right-angled side was a chord, and the hypotenuse was a chord, so it was called pythagorean theorem.

2. Sine and cosine theorem: refers to sine theorem and cosine theorem, which is an important theorem to reveal the relationship between the angles of a triangle. It can be directly used to solve the triangle problem.

3. Projective Theorem: In a right triangle, the height on the hypotenuse is the proportional average of the projections of two right-angled sides on the hypotenuse, and each right-angled side is the proportional average of the projection of this right-angled side on the hypotenuse and the hypotenuse.