说明：  对于阶数不高的方程组，直接法非常有效，对于阶数高，而系数矩阵稀疏的线性方程组却存在着困难，在这类矩阵中，非零元素较少，若用直接法求解，就要存贮大量零元素。为减少运算量、节约内存，使用迭代法更有利。
(The order of the equations is not high, the direct method is very effective for high order, and the sparse coefficient matrix of linear equations, there are difficulties in this type of matrix, the fewer non-zero elements, if the direct method , it is necessary to store a large number of zero elements. To reduce the computation, saving memory, better use of iterative method.)