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.