All formulas in mathematics have their functions. Although it was not easy to recite, I wrote it down. If you get the exam questions, you just need to memorize the formula and then enter the formula to answer them, and you will be able to answer them soon. If you don't know the formula because you have a bad back in the exam, you don't know how to answer it at all. Therefore, mathematical formula is the basis of solving mathematical problems and a general mathematical formula summarized by predecessors. As long as you find the problem corresponding to the formula, you can substitute it into the formula to solve it. Therefore, the formula that is difficult to remember in mathematics has its function, and it must be useful to remember it.