Is the limit problem of composite function. These two functions are continuous, so as long as f(x) has a limit, the equation holds. These things are not skills, but belong to the category of basic concepts. It is easy to get dizzy if you don't understand the basic concepts, but it is also an inevitable process. I think it is better to do more basic questions if the foundation is not good, and it is better to thoroughly understand the basic concepts and then study the skills.