C :

#include <stdio.h>
#include <math.h>

int main()

{
    double a,b,c,disc,x1,x2;
    while(~scanf("%lf%lf%lf",&a,&b,&c))
    {
        disc = b * b - 4 * a * c;
        if(disc > 0)
        {
            x1 = (-b - sqrt(disc)) / (2 * a);
            x2 = (-b + sqrt(disc)) / (2 * a);
            printf("x1=%.3f x2=%.3f\n",x1,x2);
        }
        else if(disc == 0)
        {
            x1 = -b / (2 * a);
            printf("x1=%.3f\n",x1);
        }
        else
        {
            double p = -b / (2 * a);
            double q = sqrt(-disc) / (2 * a);
            printf("x1=%.3f+%.3fi x2=%.3f-%.3fi\n",p,q,p,q);
        }
    }
    return 0;
}

C++ :

#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
double disc,x1,x2,p,q;
void big(int a,int b)	//>0
{
	x1=(-b+sqrt(disc))/(2.0*a);
	x2=(-b-sqrt(disc))/(2.0*a);
}
void small(int a,int b)	//<0
{
	p=-1.0*b/(2*a);
	q=sqrt(-disc)/(2*a);
}
void equal(int a,int b)	//==0
{
	x1=x2=-1.0*b/(2*a);
}
int main()
{
	int a,b,c;
	cin>>a>>b>>c;
	disc=b*b-4*a*c;
	if (disc>0)
	{
		big(a,b);
		printf("x1=%.3lf x2=%.3lf\n",x1,x2);
	}
	else if (disc==0)
	{
		equal(a,b);
		printf("x1=x2=%.3lf\n",x1);
	}
	else
	{
		small(a,b);
		printf("x1=%.3lf+%.3lfi x2=%.3lf-%.3lfi\n",p,q,p,q);
	}
	return 0;
}