The following is the introduction of auxiliary lines:

Auxiliary line refers to a line or line segment with great value made on the basis of the original drawing, which is often used to solve difficult geometric problems in geometry.

Methods 1: The midline of triangle is always double. Questions with a midpoint, usually the center line of a triangle. By this method, the conclusion to be proved is properly transferred, and the problem is easily solved.

Method 2: When there is a bisector, we often take the angular bisector as the symmetry axis, use the properties of the angular bisector and the conditions in the problem to construct a congruent triangles, and use congruent triangles's knowledge to solve the problem.

Method 3: The conclusion is that when two line segments are equal, auxiliary lines are often drawn to form congruent triangles, or some theorems about bisecting line segments are used.

Refer to Baidu Encyclopedia-Auxiliary Line for the above information.