数值方法解方程组AX=B（LU分解），过程分3步，首先，分解系数矩阵A=LU，然后，解LY=B，最后再求UX=Y 另一种是直接求解的（2.1）-Numerical method to solve the equations AX = B (LU decomposition), 3 step process, first of all, the decomposition of the coefficient matrix A = LU, then, Xie LY = B, and finally seek UX = Y　another directly solving (2.1 )