Procedure ZBRAC(var X1, X2:real; SUCCES:boolean);
var
    NTRY,J:integer;  FACTOR,F1,F2:real;
begin
    FACTOR:=1.6;
    NTRY:=50;
    If X1 = X2 Then ShowMessage('you have to guess an initial range');
    F1:=FUN(X1);
    F2:=FUN(X2);
    SUCCES:=True;
    For J:=1 To NTRY do
    begin
        If F1 * F2 < 0 Then Exit;
        If Abs(F1) < Abs(F2) Then
        begin
            X1:=X1 + FACTOR * (X1 - X2);
            F1:=FUN(X1);
        end
        Else
        begin
            X2:=X2 + FACTOR * (X2 - X1);
            F2:=FUN(X2);
        end;
    end; 
    SUCCES:=False;
end;