The foundation is not solid. Do more basic questions and start from the beginning.

If the method is improper, learn more skills and attack more skillful topics.

If you don't consider the problem comprehensively, you should always remind yourself of the trap. I think the most important thing is the limitation of the topic. When doing the purpose of the problem, always remind yourself not to think in a fixed way, and pay attention to the changes in conditions. There are many restrictions on practical problems, such as definition domain, which should be paid special attention to. To get into the habit of checking after finishing the questions, we should first consider whether it conflicts with common sense.