根本不用二分，十行代码暴力枚举就可以了。

#include <bits/stdc++.h>
using namespace std;
int main()
{
	double a, b, c, d;
	cin >> a >> b >> c >> d;
	for (double i = - 100; i <= 100; i += 0.0001) {
		double left = a * i * i * i + b * i * i + c * i + d;
		if (left < 0.0001 && left > -0.0001) cout << fixed << setprecision(2) << i << " ";
	}
	return 0;
}

建议降成红题