C :

#include<stdio.h>
#include<math.h>
int main()
{
	double a, b, c, C;
	scanf("%lf%lf%lf", &a, &b, &c);
	C = pow(b, 2) - 4 * a * c;
	if(C == 0)
		printf("x1=%.3lf x2=%.3lf\n", -b / (2 * a), -b / (2 * a));
	if(C < 0)
		printf("x1=%.3lf+%.3lfi x2=%.3lf-%.3lfi\n",-b / (2 * a), sqrt(-C) / (2 * a), -b / (2 * a), sqrt(-C) / (2 * a));
	if(C > 0)
	printf("x1=%.3lf+%.3lf x2=%.3lf-%.3lf\n",-b / (2 * a), sqrt(C) / (2 * a), -b / (2 * a), sqrt(C) / (2 * a));
}

C++ :

#include <stdio.h>
#include <cmath>
#include <string.h>
#include <cstdlib>
#include <iostream>
#define eps 1e-6
using namespace std;


int main()
{
    double a, b, c, disc, x1, x2, real, imag;
    scanf("%lf%lf%lf", &a, &b, &c);

    disc = b * b - 4 * a * c;
    if(fabs(disc) <= eps)
    {
        printf("x1=%.3lf ", -b / (2 * a));
        printf("x2=%.3lf\n", -b / (2 * a));
    }
    else if(fabs(disc > eps))
    {
        x1 = (-b + sqrt(disc)) / (2 * a);
        x2 = (-b - sqrt(disc)) / (2 * a);
        printf("x1=%.3lf x2=%.3f\n", x1, x2);
    }
    else
    {
        real = -b / (2 * a);
        imag = sqrt(-disc) / (2 * a);

        printf("x1=%.3f+%.3fi ", real, imag);
        printf("x2=%.3f-%.3fi\n", real, imag);
    }

    return 0;
}