最速下降法求解方程f=1/2x *A*x+b *x求其极小值点，其中梯度g=Ax-b,下降方向p=-g，x为初始点，eps为精度。

注：本程序只适用于二次方程，即方程的A矩阵不含未知数。-Steepest descent method for solving the equation f = 1/2x　* A* x+ b * x seeking the minimum point where the gradient g = Ax-b, the descent direction p =-g, x is the initial point, eps of precision .

Note: This procedure applies only to the quadratic equation, A matrix equation that is free of unknowns.