Tg^2(a)-sin^2(a)-tg^2(a)*sin^2(a)=(sin^2(a)-sin^2(a)*cos^2(a))/cos^2(a) - tg^2(a)*sin^2(a)=
(sin^2(a)/cos^2(a))* (1-cos^2(a)) - tg^2(a)*sin^2(a) = tg^2(a)*sin^2(a)-tg^2(a)*sin^2(a)=0
P.S Работать только с tg^2(a) - sin^2(a).
tg^2(a)-sin^2(a) = tg^2(a)*sin^2(a)