The reason is that they gave the textbook a month and a half later. As a result, when they finally did the problem, they recited the steps of solving the familiar problem completely. But they haven't seen each other since they went to the battlefield, and they asked me about the integral of the surface before the exam. They don't understand that the premise of surface integral is that the functional relationship is only defined on that surface in space. Only in the end do you find that the textbook is telling you how the content of this section came out, guiding you step by step, and letting you know the development law of the whole knowledge point, why it was put forward, under what circumstances, and what is the use.
When reading the textbook, don't scan it easily, but chew the knowledge thoroughly, so that when doing the problem, even if you don't know which step, you will know the background and purpose of the whole problem like the back of your hand, and you will be deeply touched when you look back. Personally, I think I can stay for two and a half months at most to practice many problems. Good luck.