int (tg^2 x + tg^4 x) dx = int (tg^2 x * (1 + tg^2 x)) dx =
= int (tg^2 x * (1 + sin^2 x/cos^2 x)) dx =
= int (tg^2 x * 1/cos^2 x) dx = int tg^2 x d(tg x) = | t = tg x | =
= int t^2 dt = 1/3 * t^3 + C = | t = tg x | = 1/3 * tg^3 x + C