Программа неправильно решает квадратное уравнение

//---------------------------------------------------------------------------
 
#include <vcl.h>
#pragma hdrstop
#include <stdio.h>
#include <math.h>
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
}
//---------------------------------------------------------------------------
int a,b,c;
float x1,x2,d;
//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
a=StrToInt (Edit1->Text);
b=StrToInt (Edit2->Text);
c=StrToInt (Edit3->Text);
 
d=b^2-4*a*c;
 
Label5->Caption= "Diskrimenant = " + FloatToStr(d);
 
if(d<0)
{
Label6->Caption= "Pri D<0 yravnenie ne imeet kornei.";
}
else
 
if(d==0)
{
x1=x2= -b/2*a;
Label6->Caption= "Koren nomer1=nomer2= " + FloatToStr(x1);
}
 
else
 
if(d>0)
{
x1= (-b- sqrt(d))/2*a;
x2= (-b+ sqrt(d))/2*a;
Label6->Caption= "Koren nomer1 = " + FloatToStr(x1);
Label7->Caption= "Koren nomer2 = " + FloatToStr(x2);
 
} 
 
 
}
//---------------------------------------------------------------------------

d = b * b - 4 * a * c;