1, midline: In a triangle, the line segment connecting a vertex and its relative midpoint is called the midline of the triangle.

2. The outer angle of a triangle is equal to the sum of two non-adjacent inner angles.

3. Stability of the triangle: When the lengths of the three sides of the triangle are determined, the shape and size of the triangle are determined. This property is called the stability of triangles.

4. Any finite decimal or infinite cyclic decimal is a rational number.

5. By adding and subtracting two equations, one of the unknowns is eliminated. This method of solving binary linear equations is called addition, subtraction and elimination, which is called addition and subtraction for short.