rk

说明：  用四阶古典RK方法解初值问题： 取h=1/8。每隔8步打印出数值解与真解的值（u(t)=(t^2)/2-t），画出它们的大致图像，并对产生的结果做出解释。 (with four bands classical RK method for initial value problems : from h = 1/8. Every eight-step numerical solution print and in the true value (u (t) = (t ^ 2)/2-t), they generally draw the images, as well as the results of an explanation.)