Adding the center line, in an isosceles triangle, you can usually get a lot by adding one, such as adding the center line and bisecting the angle. This is the most commonly used, you can choose to add it according to the formula, but after adding it, you should know what conclusion you can draw, and generally you should find out the congruent triangles, and it is easier to find the congruence condition according to this. When we encounter difficulties, we can tear down the simple graphics and find the basic graphics we have done before, but. It is a good idea to find out the basic graphics and add auxiliary lines as needed, rather than blindly adding them, otherwise the more you think about it, the more difficult it will be. If you have an angle, you must be vertical. In isosceles, if you want to combine three lines.