Выражения 1)sin^2 a tg^2 a + cos^ a 2)sin^4 a - cos^4 a + cos ^2 a

2) = (sin^2 a - cos^2 a)(sin^2 a + cos ^2 a) + cos^2 a= sin^ 2 a - cos ^ 2 a + cos ^ 2 a= sin^ 2 a.