Function RTBIS(X1, X2, XACC:real):real;
Label 99;
var
    DX,FMID,F,XMID,RTB:real;   J,JMAX:integer;
begin
    JMAX:=40;
    FMID:=FUN(X2);
    F:=FUN(X1);
    If F * FMID >= 0  Then
        ShowMessage('Root must be bracketed for bisection.');
    If F < 0  Then
    begin
        RTB:=X1;
        DX:=X2 - X1;
    end
    Else
    begin
        RTB:=X2;
        DX:=X1 - X2;
    end;
    For J:=1 To JMAX do
    begin
        DX:=DX * 0.5;
        XMID:=RTB + DX;
        FMID:=FUN(XMID);
        If FMID <= 0 Then RTB:=XMID;
        If (Abs(DX) < XACC) Or (FMID = 0) Then goto 99;
    End; 
    ShowMessage('too many bisections');
99: RTBIS:=RTB;
end;