赖皮公式代入法：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>

double a, b, c, d, cnt = 0;
double as, bs, t, si;
double x1, x2, x3;
int main()
{
	std::cin >> a >> b >> c >> d;
	as = b * b - 3 * a * c;
    bs = b * c - 9 * a * d;
    t = (2 * as * b - 3 * a * bs) / (2 * sqrt(as * as * as));
    si = acos(t);
    x1 = (- b - 2 * sqrt(as) * cos(si / 3)) / (3 * a);
    x2 = (- b + sqrt(as) * (cos(si / 3) + sqrt(3) * sin(si / 3))) / (3 * a);
    x3 = (- b + sqrt(as) * (cos(si / 3) - sqrt(3) * sin(si / 3))) / (3 * a);
    if(x1 > x2)
    {
    	double t = x1; x1 = x2; x2 = t;
    }
    if(x1 > x3)
    {
    	double t = x1; x1 = x3; x3 = t;
    }
    if(x2 > x3)
    {
    	double t = x2; x2 = x3; x3 = t;
    }
    printf("%.2f %.2f %.2f", x1, x2, x3);
	return 0;
}