I tell you, the knowledge of mathematics is not available in books, so you should study it. The method of solving problems is wonderful, not a theorem. You may think it's too simple, so you seem to understand in class, but you don't.

The solution is to do more examples and study other people's solutions, which is very important. There is no topic you don't know at all. Step by step, after you fully understand the simple ones and can draw inferences from others, do the complicated ones. You will certainly understand them.