Secondly, when doing problems, we should cultivate a lazy spirit and consciousness, use more brains and do less work, that is, try to find the least way to write. Ps: except analytic geometry. (Very thoughtful)

Thirdly, the reason for cutting strings is that there are few theorems about cutting lines, cos, sin? Many, so see tan, second class. Crime first, cos.

Fourth, when solving problems, we can deduce from the conclusion of verification, because the conclusion can give people a hint.

Fifth, add several formulas.

1. product and difference? : 8080/htmlfile/p200310713566.htm (with details)

2. General formula (as shown in the figure)

Six-ly, a nonsense: analytic geometry uses the basic theorems: vertical diameter and line-cutting theorem, which can greatly reduce the amount of calculation.