There are many formulas in electromagnetism, but only a few formulas need to be memorized, such as electrostatic force formula, Gauss theorem and Pisa theorem. Most of the others can be deduced from these basic theorems (or basically written soon). I suggest that the study should echo before and after, so that after forming a relatively complete theoretical system, many formulas are actually not so difficult to draw. Of course, we should lay a solid foundation in mathematics.