The key of partial derivative lies in understanding. One is the definition, which is actually a fixed derivative from X to Y and a fixed derivative from Y to X. Understanding the definition is simple. In addition, it is also a common thing to master the law of chain derivatives. The limit, derivative, integral and mean value theorems of high numbers in front are really difficult, and the multivariate behind them is more about calculation and mastery of rules. The key is to understand.

If you want a series, you should first build a framework. What are the definitions of several series? It can be divided into positive series, staggered series and arbitrary series. What is the judgment method of convergence of all levels? How to find the limit with sequence? How to judge the convergence domain and radius of ghost series? Then it expands and contracts.

My friends, the real difficulties really lie ahead.

If you really don't understand, you can watch some videos. Looking for Zhang Yu's video should help you understand.

However, my friend, this is not to discourage you. If you don't even understand the basic concepts now, I think you need to refuel.