C :

#include<stdio.h>
#include<math.h>
void f1(int a,int b,int c);
void f2(int a,int b,int c);
void f3(int a,int b,int c);

int main()
{
	int s;
	unsigned int a,b,c;
	scanf("%d %d %d",&a,&b,&c);
	if(a==0)
		printf("not a quadratic\n");
	s=b*b-4*a*c;
	if(s>0)
		f1(a,b,c);
		else if(s==0)
			f2(a,b,c);
		else
			f3(a,b,c);
	return 0;
}

void f1(int a,int b,int c)
{
	double k,x1,x2;
	k=sqrt(b*b-4*a*c);
	x1=(-b+k)/(double)(2*a);
	x2=(-b-k)/(double)(2*a);
	printf("distinct real roots:%.4lf %.4lf\n",x1,x2);
}

void f2(int a,int b,int c)
{
	double x;
	x=-b/(double)(2*a);
	printf("two equal roots:%.4lf\n",x);
}

void f3(int a,int b,int c)
{
	double k,x1,x2;
	k=sqrt(4*a*c-b*b);
	x1=-b/(double)(2*a);
	x2=k/(double)(2*a);
	printf("complex roots:%.4lf+%.4lfi %.4lf-%.4lfi\n",x1,x2,x1,x2);
}

C++ :

#include <stdio.h>
#include <math.h>
int main() {
	void t1(int a, int b, int c);
	void t2(int a, int b, int c);
	void t3(int a, int b, int c);
	int a, b, c, delta;
	double ans0, ans1, sqrt_delta, realpart, imagpart;
	scanf("%d %d %d", &a, &b, &c);
	/* 计算delta的值（b*b-4*a*c）并分类讨论 */
	delta = b * b - 4 * a * c;
	if (delta > 0) {
		t1(a, b, c);
	} else if (delta == 0) {
		t2(a, b, c);
	} else {
		t3(a, b, c);
	}
	return 0;
}
void t1(int a, int b, int c) {
	int delta;
	double sqrt_delta, ans0, ans1;
	delta = b * b - 4 * a * c;
	sqrt_delta = sqrt(delta);
	ans0 = (-b - sqrt_delta) / (2.0 * a);
	ans1 = (-b + sqrt_delta) / (2.0 * a);
	printf("distinct real roots:%.4f %.4f\n", ans0, ans1);
}
void t2(int a, int b, int c) {
	double ans0;
	ans0 = (-b) / (2.0 * a);
	printf("two equal roots:%.4f\n", ans0);
}
void t3(int a, int b, int c) {
	int delta;
	double realpart, imagpart;
	delta = b * b - 4 * a * c;
	realpart = -b / (2.0 * a);
	imagpart = sqrt(-delta) / (2.0 * a);
	printf("complex roots:%.4f+%.4fi %.4f-%.4fi\n", realpart, imagpart, realpart, imagpart);
}

Pascal :

var 
  a,b,c:longint;
  d:double;

procedure fa;
begin
  writeln('not a quadratic');
end;

procedure fb;
begin
  writeln('two equal roots:',-b/(2*a):0:4);
end;

procedure fc;
begin
  writeln( 'distinct real roots:', 
    (-b-sqrt(d))/(2*a):0:4 ,' ', (-b+sqrt(d))/(2*a):0:4 );
end;

procedure fd;
begin
  writeln( 'complex roots:', 
    (-b)/(2*a):0:4,'+',sqrt(-d)/(2*a):0:4,'i ',
    (-b)/(2*a):0:4,'-',sqrt(-d)/(2*a):0:4,'i' );
end;

begin
  readln(a,b,c);
  d:=b*b-4*a*c;
  if( a<1e-8 ) then fa
    else if( (d>0) and (d<=1e-8) ) then fb
      else if( d>1e-8 ) then fc
        else if(d<0) then fd;
end.