[AND_REY]

Так попробуй:

Код:

Uses Math;
procedure TForm1.Button1Click(Sender: TObject);
Var
 x, Xmin, Xmax, e, R: Real;
begin
 Xmin:= StrToFloat(Edit1.Text);
 Xmax:= StrToFloat(Edit2.Text);
 e:= StrToFloat(Edit3.Text);
 x:= Xmin;
 Repeat
  R:= 3*Tan(x)-1/3;
  x:= x + 0.001;
  if Abs(R) <= e Then Label1.Caption:= 'X = '+FloatToStr(x);
 Until x >= Xmax;
end;