tg^3x-tg^3y/(1+tgx*tgy)*(tg^2x+tgx*tgy+tg^2y)
(tg^3x–tg^3y)/((1+tgx·tgy)·(tg^2x+tgx·tgy+tg^2y))= =((tgx-tgy)*(tg^2x+tgx·tgy+tg^2y))/((1+tgx·tgy)·(tg^2x+tgx·tgy+tg^2y))= =(tgx-tgy)/(1+tgx·tgy)= =tg(x-y)