In former times parabolas could be drawn with instruments like the one in the applet.[br][br][b]Exercise[/b][br][list][br][*]Switch on the trace of point P and move point S.[br][*]Show the parabola which is drawn with the current adjustment.[br][*]Explane why the curve is a parabola.[br][*]Move focus F and watch how the form of the parabola changes.[br][/list]