说明：  割线法的理论：为了避免在牛顿法中每步迭代都要计算导函数的值，我们用相邻两步的函数值作差商来逼近导数值
(Secant theory: in order to avoid the Newton method, each iteration should calculate the value of the derivative function, we use the adjacent two-step function values ​ ​ for difference quotient to approximate the value of derivative)