1) tg^2a - sin^2a = tg^2a - (1 - cos^2a) = tg^2a - 1 + cos^2a = tg^2a - 1 + 1 - tg^2a = 0

2) 1 - tg^2a + tg^4a/cos^2a = 1 - tg^2a + tg^2a * tg^2a = 1 - tg^2a + tg^2a = 1