证明:区间套方法。由已知条件,对$a,b,\exists y_1,y_2,s.t.|f(y_2)|\leq \frac{1}{2}|f(b)|,|f(y_1)|\leq \frac{1}{2}|f(a)|,$
$\Rightarrow 0\leq ||f(y_2)|-|f(y_1)||\leq \frac{1}{2}||f(b)|-|f(a)||,$
再对$y_1,y_2,\exists y_3,y_4,s.t.|f(y_4)|\leq \frac{1}{2}|f(y_2)|,|f(y_3)|\leq \frac{1}{2}|f(y_1)|,$
$\Rightarrow 0\leq ||f(y_4)|-|f(y_3)||\leq \frac{1}{2}||f(y_2)|-|f(y_1)||\leq \frac{1}{2^2}||f(b)|-|f(a)||,$
同理,相同的办法,一直继续下去,有
$\exists y_k,y_{k+1},s.t.0\leq ||f(y_{k+1})|-|f(y_k)||\leq \frac{1}{2^k}||f(b)|-|f(a)||,$
$\therefore \Rightarrow ||f(y_{k+1})|-|f(y_k)||\rightarrow 0,(k\to \infty )$
$\therefore f(y_{k+1})=f(y_k)=0.(k\to \infty )$
令最终的点为$\xi$.此时,$f(\xi)=0.$