Function RTNEWT(X1, X2, XACC:real):real;
Label 99;
var
    J,JMAX:integer;  DX,RTN,F,DF:real;
begin
    JMAX:=20;
    RTN:=0.5 * (X1 + X2);
    For J:=1 To JMAX do
    begin
        FUND(RTN, F, DF);
        DX:=F / DF;
        RTN:=RTN - DX;
        If (X1 - RTN) * (RTN - X2) < 0 Then
        begin
            ShowMessage('jumped out of brackets');
            Exit;
        end; 
        If Abs(DX) < XACC Then goto 99;
    end; 
    ShowMessage('RTNEWT exceeding maximum iterations.');
99: RTNEWT:=RTN;
end;