利用常用的四阶龙格-库塔公式求初值再分别利用米尔恩公式和改进的亚当斯方法及常用的四阶龙格-库塔公式求解其余的数值解，并计算它们与精确解的误差-By use of Runge- Kutta formula and then find the initial value, respectively, and improved the formula by Milne Adams methods and commonly used Runge- Kutta formula to solve the rest of the numerical solution, and calculate their error with the exact solution