876AC代码，之前的题解都是错的

//需注意：当b*b-4*a*c<0时,没有答案
#include<iostream>
#include<iomanip>
#include<stdio.h>
#include<math.h>
using namespace std;
int main(){
	double a,b,c,x,y;
	cin>>a>>b>>c;
	if(a==0||b*b-4*a*c<0){
		cout<<"No answer!";
		return 0;
	}
	x=(-b+sqrt(b*b-4*a*c))/(2*a);
	y=(-b-sqrt(b*b-4*a*c))/(2*a);
	if(fabs(x-y)<1e-6){
		printf("x1=x2=%.5lf",x);
		return 0;
	} 
	if(x>y)swap(x,y);
	printf("x1=%.5lf;x2=%.5lf",x,y);
	return 0;
}

#include <stdio.h>
#include <math.h>
#include <iostream>
using namespace std;
int main()
{
	double a,b,c;
    cin >> a >> b >> c;
    if (a==0)
    {
        cout<<"No answer!";
    }
    else
    {
        double x1,x2;
        x1 = (-b + sqrt(b*b - 4*a*c))/(2*a);
        x2 = (-b - sqrt(b*b - 4*a*c))/(2*a);
        if( fabs(x1-x2)< 1e-6)
        {
            printf("X1=X2=%.5lf\n",x1);
        }
        else
        {
            if(x1 > x2)
            {
                swap(x1,x2);
            }
            printf("x1=%.5lf;x2=%.5lf\n",x1,x2);
        }
    }

}

#include<bits/stdc++.h> using namespace std; int main(){ for(long long i=0;i<=514514;i++){ cout<<"1111111111111111111111111111111111111111111111111"; } return 0; }

#include <stdio.h>
#include <math.h>
#include <iostream>
using namespace std;
int main()
{
	double a,b,c;
    cin >> a >> b >> c;
    if (a==0)
    {
        cout<<"No answer!";
    }
    else
    {
        double x1,x2;
        x1 = (-b + sqrt(b*b - 4*a*c))/(2*a);
        x2 = (-b - sqrt(b*b - 4*a*c))/(2*a);
        if( fabs(x1-x2)< 1e-6)
        {
            printf("X1=X2=%.5lf\n",x1);
        }
        else
        {
            if(x1 > x2)
            {
                swap(x1,x2);
            }
            printf("x1=%.5lf;x2=%.5lf\n",x1,x2);
        }
    }

}

真题解

https://jingyan.baidu.com/article/ca00d56c2e0952e99eebcf86.html

不会一元二次的人一定要看

完整代码如下：

#include <stdio.h>
#include <math.h>
#include <iostream>
using namespace std;
int main()
{
	double x1,x2;
	double a,b,c;
    cin >> a >> b >> c;
    if ((b * b - 4 * a * c) < 0)
    {
        cout<<"No answer!";
    }
    else
    {
        if((b * b - 4 * a * c) == 0)
        {
        	x1 = -1 * (b / (2 * a));
            printf("X1=X2=%.5lf\n",x1);
        }
        else
        {
        	x1 = (-b + sqrt(b*b - 4*a*c))/(2*a);
        	x2 = (-b - sqrt(b*b - 4*a*c))/(2*a);
            if(x1 > x2)
            {
                swap(x1,x2);
            }
            printf("x1=%.5lf;x2=%.5lf\n",x1,x2);
        }
    }

}

#include <iostream> #include <math.h> #include<iomanip> using namespace std; int main() { double x,y,a,b,c; cin>>a>>b>>c; if (a==0) { cout<<"No answer!"; return 0; } x=(sqrt(bb-4ac)-b)/(2a); y=(-sqrt(bb-4ac)-b)/(2a); if (x>y){ swap(x,y); } if (fabs(x-y)<1e-6) { cout<<"x1=x2="<<fixed<<setprecision(5)<<x<<endl; } else { cout<<"x1="<<fixed<<setprecision(5)<<x<<";x2="<<fixed<<setprecision(5)<<y<<endl; } }

#include <stdio.h>
#include <math.h>
#include <iostream>
using namespace std;
int main()
{
	double a,b,c;
    cin >> a >> b >> c;
    if (a==0)
    {
        cout<<"No answer!";
    }
    else
    {
        double x1,x2;
        x1 = (-b + sqrt(b*b - 4*a*c))/(2*a);
        x2 = (-b - sqrt(b*b - 4*a*c))/(2*a);
        if( fabs(x1-x2)< 1e-6)
        {
            printf("X1=X2=%.5lf\n",x1);
        }
        else
        {
            if(x1 > x2)
            {
                swap(x1,x2);
            }
            printf("x1=%.5lf;x2=%.5lf\n",x1,x2);
        }
    }

}

#include <iostream>
#include <math.h>
#include<iomanip>
using namespace std;
int main()
{
    double x,y,a,b,c;
    cin>>a>>b>>c;
    if (a==0)
    {
        cout<<"No answer!";
        return 0;
    }
    x=(sqrt(b*b-4*a*c)-b)/(2*a);
    y=(-sqrt(b*b-4*a*c)-b)/(2*a);
    if (x>y){
        swap(x,y);
    }
    if (fabs(x-y)<1e-6)
    {
        cout<<"x1=x2="<<fixed<<setprecision(5)<<x<<endl;
    }
    else
    {
        cout<<"x1="<<fixed<<setprecision(5)<<x<<";x2="<<fixed<<setprecision(5)<<y<<endl;
    }
}