说明：  常微分方程的数值解和精确解，数值解法包括Euler方法，改进的Euler方法，2阶R-K方法，4阶经典R-K方法
(Numerical solution and exact solution of ordinary differential equations, numerical solutions include Euler method, the improved Euler method, 2 order R- K method, 4 order classical R- K method)