Truncation is an auxiliary line method commonly used in triangle congruence proof;

Truncation: Intercept one of the longer segments, which is equal to one of the other two segments, and then prove that the rest is equal to the other segment; ?

Supplement: extend a shorter line segment, and the extended part is equal to another shorter line segment, and then prove that the new line segment is equal to a longer line segment; Or the extension line of a shorter line segment is equal to a longer line segment, and then it is proved that the extension line is equal to another shorter line segment.

Generally speaking, the following situations need to be considered. When the quantitative relationship between two line segments mentioned above and the sum and difference relationship between three or four line segments appear, we can add auxiliary lines by truncation method;

When this quantitative relationship appears in the topic condition, you can also add auxiliary lines by cutting the length; In fact, the truncation method can also be used to prove that the sum of two angles is equal to 180.

The midpoint is a special point in a geometric figure, which appears in the figure. What figures have we learned about the properties of midpoint correlation? (1) isosceles triangle with three lines in one;

(2) The median line on the hypotenuse of the right triangle is equal to half of the hypotenuse;

(3) Figure-8 congruent figure.

The midpoint may also be related to central symmetry. The key to solve the midpoint problem is to add auxiliary lines appropriately through association, such as double long midline, midline on the hypotenuse of right triangle, midline of triangle, and central symmetrical figure.