procedure TForm1.DBGrid1CellClick(Column: TColumn);
begin
    if dbgrid1.SelectedIndex<>2 then exit;
    if not (ADOTable1.State in [DsEdit,DsInsert]) then
        ADOTable1.Edit;
    DBGrid1.Fields[2].Value:=DBGrid1.Fields[0].AsFloat+DBGrid1.Fields[1].AsFloat;
    ADOTable1.Post;
end;