In 48:15 of this video, I don't understand what the product have to do with the problem 4 and what that product is meaning. Link: http://www.youtube.com/watch?feature=player_detailpage&list=PL6763F57A61FE6FE8&v=R_gDV17X7pc#t=2880s
Show that with $e(h)=(\frac{1-h^2}{1+h^2}, \frac{2h}{1+h^2})$, $e(h_1)*e(h_2)=e(h_3)$ where $h_3=\frac{h_1 + h_2}{1-h_1h_2}$, also, any interesting background or properties about this problem or any of its significance?