In the investigation of mathematical thought, the core of transforming thought is to turn the unknown into the known and the complex into the simple. Similarly, the projective theorem and the circular power theorem are only a conclusion obtained by similar triangles's further application, and we can also regard them as different variants or deformations of a basic graph. How many theorems should be listed in the textbook just because there are many variations? I'm sure no one will think so. However, like the last example, since it can be completely proved by students, what are the problems in the practice of increasing students' experience? Knowledge and methods are like eating delicious food, and the process of chewing is also an experience. On the contrary, it is not necessarily good to make food into rice paste and drink soup.

It can be seen that it is very simple to prove these primitive theorems by similarity.

The deletion of the theorem of the approved textbook, through the experience of exercises, does not mean dilution, because the investigation of the senior high school entrance examination has never been weakened, and any senior high school entrance examination paper is absolutely indispensable, so you can go and investigate it. After increasing experience, students should be proficient in making their own conclusions and paving the way for solving deeper problems.

Fourthly, the explanation of this problem can also be explained by the internal angle and theorem of triangle. The inference of the theorem-the external angle theorem is familiar to everyone, but the further expansion of the two will lead to more basic figures and conclusions, which will no longer appear in the form of theorems or inferences in textbooks.