Вычисление выражения и ошибка "sqrt DOMAIN error"

//---------------------------------------------------------------------------
#include <vcl.h>
#include <math.h>
#pragma hdrstop
 
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BitBtn1Click(TObject *Sender)
{
 
float x,y;
float PX,PY;
float t;
const l = 1;
for (t=0.4;t<=1.4;t+=0.1)
{
for (PX=0.4;PX<=Image2->Width;PX++)
{
x= PX*10/Image2->Width;
y = ((sqrt(-1*pow(l,2)*cos(x))) - cos(t))/(cos(2*x));;
PY=Image2->Height/2-y*Image2->Height/2;
Image2->Canvas->LineTo(PX,PY);
}
}
}