где ошибка?

var
  Form4: TForm4;
  j,i,n:integer;x1,x2,x3,x4,y1,y2,y3,y4:integer;r,a,b:integer;c:real;
 
implementation
 
{$R *.dfm}
 
    procedure TForm4.Button1Click(Sender: TObject);
begin
   Form4.Timer1.enabled:=true;
 form4.Timer1.interval:=500;
end;
 
procedure TForm4.Timer1Timer(Sender: TObject);
begin
 x1:=40;
y1:=50;
x2:=90;
y2:=100;
x3:=40;
y3:=75;
x4:=90;
y4:=75;
a:=0;
r:=10;
  While (a<>r) do
 begin
  form4.Canvas.Pen.Color:=clRed;
form4.Canvas.Brush.Color:=clyellow;
  Form4.Canvas.Pie(x1,y1,x2,y2,x3,y3,x4,y4);
form4.Canvas.Brush.Color:=clBtnFace;
form4.Canvas.Pen.Color:=clBtnFace;
  Form4.Canvas.Pie(x1,y1,x2,y2,x3,y3,x4,y4);
  a:=a+1;
 c:=Sqrt(abs(sqr(r)-sqr(a)));
  b:=round(c);
    x1:=x1+a;
    y1:=y1+b;
      x2:=x2+a;
    y2:=y2+b;
     x3:=x3+a;
    y3:=y3+b;
     x4:=x4+a;
    y4:=y4+b;
    end;
  While (a<>0) do
 begin
   form4.Canvas.Pen.Color:=clRed;
form4.Canvas.Brush.Color:=clyellow;
  Form4.Canvas.Pie(x1,y1,x2,y2,x3,y3,x4,y4);
form4.Canvas.Brush.Color:=clBtnFace;
form4.Canvas.Pen.Color:=clBtnFace;
  Form4.Canvas.Pie(x1,y1,x2,y2,x3,y3,x4,y4);
 a:=a-1;
 c:=Sqrt(abs(sqr(r)-sqr(a)));
  b:=round(-c);
    x1:=x1+a;
    y1:=y1+b;
      x2:=x2+a;
    y2:=y2+b;
     x3:=x3+a;
    y3:=y3+b;
     x4:=x4+a;
    y4:=y4+b;
  end;
  While (x1<>40) do
  begin
   form4.Canvas.Pen.Color:=clRed;
form4.Canvas.Brush.Color:=clyellow;
  Form4.Canvas.Pie(x1,y1,x2,y2,x3,y3,x4,y4);
form4.Canvas.Brush.Color:=clBtnFace;
form4.Canvas.Pen.Color:=clBtnFace;
  Form4.Canvas.Pie(x1,y1,x2,y2,x3,y3,x4,y4);
  x1:=x1-2;
  x2:=x2-2;
  x3:=x3-2;
  x4:=x4-2;
  end;
end;

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, StdCtrls;
 
type
  TForm1 = class(TForm)
    Timer1: TTimer;
    Button1: TButton;
    procedure Timer1Timer(Sender: TObject);
    procedure Line(x, y,x1,y1,a,col: integer);
    procedure LineDel(x, y,x1,y1, a, col: integer);
 
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
 
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
var
  Form1: TForm1;
  a: integer;
implementation
 
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
 Timer1.Enabled:= True;
 Timer1.Interval:= 50;
end;
 
 
procedure TForm1.FormCreate(Sender: TObject);
begin
a:= 0;
 Timer1.Enabled:= False;
end;
 
procedure TForm1.Line(x, y,x1,y1,a, col: integer);
const
G= 0.0174532;
 var
x2,y2,x3,y3: integer;
begin
 Canvas.Pen.Color:= col;
 Canvas.Brush.Color:= col;
 x2:= Round(Round((x+x1)/2)+x*cos(a*G));
 y2:= Round(Round((y+y1)/2)+y*sin(a*G));
 x3:= Round(Round((x+x1)/2)-x*cos(a*G));
 y3:= Round(Round((y+y1)/2)-y*sin(a*G));
 Canvas.Pie(x,y,x1,y1,x2,y2,x3,y3);
end;
 
 
procedure TForm1.LineDel(x, y,x1,y1, a, col: integer);
const
G= 0.0174532;
var
 x2,y2,x3,y3: integer;
begin
 Canvas.Pen.Color:= col;
 Canvas.Brush.Color:= col;
 x2:= Round(Round((x+x1)/2)+x*cos(a*G));
 y2:= Round(Round((y+y1)/2)+y*sin(a*G));
 x3:= Round(Round((x+x1)/2)-x*cos(a*G));
 y3:= Round(Round((y+y1)/2)-y*sin(a*G));
 Canvas.Pie(x,y,x1,y1,x2,y2,x3,y3);
end;
 
 
procedure TForm1.Timer1Timer(Sender: TObject);
begin
 Form1.LineDel(100,100,200,200,a,clBtnFace);
  a:=a+1;
 Form1.Line(100,100,200,200,a,clRed);
 
end;
 
end.