Помогите, пожалуйста, найти
int x * sin x/cos^3 x dx
int x * sin x/cos^3 x dx = int x/cos^3 x d(-cos x) = -int x/cos^3 x d(cos x) =
= -int x d(-1/2 * 1/cos^2 x) = 1/2 * int x d(1/cos^2 x) =
= 1/2 * x * 1/cos^2 x - 1/2 * int 1/cos^2 x dx = 1/2 * x * 1/cos^2 x - 1/2 * tg x + C
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