P1024 一元三次方程求解

https://www.luogu.org/problem/P1024

#include<iostream>
#include<cstdio>
#include<cmath>
#define eps 1e-4
using namespace std;
double x1,x2,x3,a,b,c,d;
double f(double x){return a*x*x*x+b*x*x+c*x+d;}
double df(double x){return 3*a*x*x+2*b*x+c;}
double slove(double l,double r)
{
    double x,x0=(l+r)/2;
    while(abs(x0-x)>eps)
      x=x0-f(x0)/df(x0),swap(x0,x);
    return x;
}
int main()
{
    cin>>a>>b>>c>>d;
    double p=(-b-sqrt(b*b-3*a*c))/(3*a);
    double q=(-b+sqrt(b*b-3*a*c))/(3*a);
    x1=slove(-100,p),x2=slove(p,q),x3=slove(q,100);
    printf("%.2lf %.2lf %.2lf",x1,x2,x3);
    return 0;
}