Properties of equation 1: The equation still holds when the same number (formula) is added (subtracted) on both sides of the equation.

Properties of equation 1: Multiply (divide) the same number (formula) whose two sides are not zero at the same time, and the equation still holds.

Equivalence relation related to triangle:

1.

The sum of the internal angles of the triangle = 180.

2. Pythagorean theorem: a?

+

b？ =c？

I don't know if I have learned something: such as trigonometric function equivalence relation. ...

Unequal relations related to triangles:

1. The sum of two sides is greater than the third side.

2. The difference between the two sides is smaller than the third side.