High school knowledge is that practice makes perfect. You have seen more and accumulated more. When you see this kind of problem, you will try to accumulate it in various ways. This is most obvious in mathematics, among which series and conic curve are the most representative. Pre-study and post-review are just to exercise this kind of thinking ability and accumulate ideas for solving problems. Remember and understand that it is constant and changeable. I hope it will be helpful to your study. I entered the college entrance examination for Mathematics 132, Chongqing in 2009.