It is because many useful things have been deleted from the textbooks for solving geometric problems, such as the three sine theorem, the three cosine theorem, the three perpendicular theorem, the three corner cosine theorem, the three corner sine theorem, the area projection theorem, and some other formulas, such as the angle formed by non-planar straight lines and the distance formula of non-planar straight lines, which are all accumulated after class. You can choose to fill in the blanks directly during the exam, and you can write clearly with XXX theorem when you solve the problem. ...