Trigonometric functions and series, especially series, lie in how to connect all formulas in series, how to master the variation between formulas, and what are the benefits after the variation (that is, the starting point for solving problems)? Knowing this, you can keep changing and the vector will be simpler. You can master and deeply understand the definition of vector, know the letters of direction, especially the representation method of coordinates, and convert them into vector calculation according to the meaning of the question. . . . . .

In short, after finishing high school, I found that all science subjects are a principle: to deeply understand the definition, almost all the questions are about the definition. Change all the definitions and use them skillfully, and you can break everything. The most important thing for you is all the definitions, the solutions and ideas of the examples in the book, and the solutions and ideas of the exercises after class. When you have read all the books, haha, you will know what realm is.