Use of sub

代码说明：

用对分法求非线性方程f(x)=0的实根.基本思想：逐步缩小这个有根的区间，当这个区间长度减小到一定程度是，就取这个区间的中点作为根的近似值-Use of sub-method of nonlinear equation f (x) = 0 the real roots. The basic idea: to gradually narrow the range of this well-established, when the interval length decreases to a certain extent, we take the midpoint of this range as the root approximation