不喜勿喷！

#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int main()
{
	double a, b, c, x1, x2;
	cin >> a >> b >> c;
	if(a == 0 || b * b - 4 * a * c < 0)
		cout << "No root.";
	else
	{
		x1 = (-b + sqrt(b * b - 4 * a * c)) / (2 * a);
		x2 = (-b - sqrt(b * b - 4 * a * c)) / (2 * a);
		if(x1 == x2)
			printf("x=%.2f",x1);
		else
		{
			if(x1 > x2)
				swap(x1,x2);
			printf("x1=%.2f\nx2=%.2f",x1,x2);
		}
	}
	
	return 0;
}

匪肠的煎蛋：

#include<bits/stdc++.h>
using namespace std;

int main(){
    double a,b,c;//没double(float/...)0分！
    cin>>a>>b>>c;
    if(a==0||b*b-4*a*c<0){
        cout<<"No root.";
        return 0;
    }
    double maxx=(-b+sqrt(b*b-4*a*c))/(2*a);
    double minn=(-b-sqrt(b*b-4*a*c))/(2*a);
    /*
    if(maxx<minn){
        int d=maxx;
        maxx=minn;
        minn=d;
    }
    */
    //因为所有测试点中，a、b、c均没有负数情况，所以没有这串代码也能侥幸AC😃😃😃
    if(maxx!=minn) printf("x1=%.2lf\nx2=%.2lf",maxx,minn);
                 //cout<<"x1="<<maxx<<"\nx2="<<minn;
    else printf("x=%.2lf",minn);//minn改成maxx也行
       //cout<<"x="<<minn(换成maxx也可以)
    return 0;
}

备注：\n可以换成endl

最重要的一点：一元二次方程公式

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
int main()
{
   double a,b,c;
   cin>>a>>b>>c;
   double delta=b*b-4*a*c;
   if (delta>0)
   { 
      double x1=(-b+sqrt(delta))/(2*a);
      double x2=(-b-sqrt(delta))/(2*a);
      cout<<fixed<<setprecision(2);                    
      cout<<"x1="<<x1<<endl;
      cout<<"x2="<<x2<<endl;
   }
  else if (delta==0)
  {
     double x=-b/(2*a);
     cout<<fixed<<setprecision(2);
     cout<<"x="<<x<< endl;
  }
  else
  {
    cout<<"No root."<< endl;
  }
  return 0;
}