Generally, triangles are congruent, straight lines are parallel, and the nature of a circle: the diagonal of a quadrilateral is 1 degree, the angles corresponding to two arcs are equal, and so on. And the simple nature of trapezoid. In fact, in junior high school, we usually use the method of guessing first and then proving to find the connection between angles. If the things to be proved are far away, it is generally not difficult to make auxiliary lines, which are all in formula mathematics books.