用Doolittle方法求解此方程组； 1．先由U[1][i]=a[1][i] 求的U的第一行，在根据L[i][1]=a[i][1]/U[1][1] 求的L的第一列；2..然后根据公式求得U，L其他的值； 3 回带求解得到X -Try using this method to solve equations; 1. First U [1] [i] = a [1] [i] for the U in the first line, in accordance with L [i] [1