二分法求多项式单根(20)

#include <iostream>
using namespace std;
double a3, a2, a1, a0;
double fun(double x);
int main()
{
    double a, b, mid;
    cin >> a3 >> a2 >> a1 >> a0;
    cin >> a >> b;
    do{
        if ((b-a)*1000<1.0){
            printf("%.2lf", (a+b)/2);
            break;
        }
        mid = (a + b) / 2;
        if (fun(mid) ==0){
            printf("%.2lf", mid);
            break;
        }
        else if (fun(mid)*fun(a)>0){
            a = mid;
        }
        else{
            b = mid;
        }
    } while (true);
}
double fun(double x)
{
    return a3*x*x*x + a2*x*x + a1*x + a0;
}