已知初始时刻的R0,V0；求tao时间后的R和V(思路：先求轨根，再求t时刻后的f,再由轨根求位置速度)-Known to the initial time of R0, V0　seeking tao time after the R and V (thinking: first seeking the root, and then t after the f, and then by the rail to find the location speed)