(1) Parallel lines are a basic figure: when parallel lines appear in geometry, the key to adding auxiliary lines is to add a third line intersecting with the two parallel lines.

(2) An isosceles triangle is a simple basic figure: when there are two equal line segments from one point in a geometric problem, it is often necessary to complete the isosceles triangle. When the combination of bisector and parallel line appears, the intersection of two sides of parallel line and angle can extend to form an isosceles triangle.

(3) The important line segment in the isosceles triangle is an important basic figure: the midpoint on the bottom of the isosceles triangle is added with the midline on the bottom; When the bisector of the angle is combined with the vertical line, when the vertical line intersects with the two sides of the angle, the basic figure of the important line segment in the isosceles triangle can be extended.

(4) The basic figure of the midline on the hypotenuse of the right triangle: the midpoint on the hypotenuse of the right triangle is often added with the midline on the hypotenuse. If the line segment is the hypotenuse of a right triangle, it is necessary to add the midline to the hypotenuse of the right triangle to get the basic figure of the midline on the hypotenuse of the right triangle.

(5) Basic figure of triangle midline: When there are multiple midpoints in the geometric problem, the basic figure of triangle midline is often added to prove it; When there is a midpoint but no midline, increase the midline; When there is a midpoint, the triangle needs to be completed; When there is a line segment folding relationship, and the line segment with the same end point has a midpoint, the parallel lines folded by the line segment can be added through the midpoint to get the basic figure of the triangle midline; When there is a line segment folding relationship, and the endpoint of the line segment is the midpoint of a line segment, the basic figure of the triangle midline can be obtained by adding the parallel lines of the line segment with the midpoint.

(6) congruent triangles: congruent triangles has axial symmetry, central symmetry, rotation and translation; If two equal line segments or two equal angles are symmetrical about a straight line, you can add an axis symmetry to form an congruent triangle: either add an axis symmetry or flip a triangle along the axis symmetry. In geometric problems, when one or two groups of equal-length line segments are located on both sides of a pair of vertex angles and on a straight line, congruent triangles with symmetrical center can be added to prove it. Addition is to connect four endpoints in pairs or add parallel lines through two endpoints.