Ordinary differential equation. The basic idea of solving this kind of problems is very similar to that of solving equations in elementary mathematics. It is also to find out the relationship between known function and unknown function in the studied problem, and get the expression of unknown function from one or several listed equations containing unknown function. When the theory of differential equation is gradually improved, it can accurately express the basic laws of things' changes. As long as the corresponding differential equations are listed, there is a way to understand them. Differential equations have become the most important branch of mathematics.