In the fifth section, approximate calculation is rarely tested, differential equations have been tested before, and Euler formula has been tested for small problems (the operator method is easier to handle)

In the sixth section, I don't know how to take the exam alone, but it is very useful to judge some multiple-choice questions. I can't help but look at them.

The complex form of Section 8 can be omitted, and the 2l period is very important (which is also the focus of Fourier).