int x * cos x/sin^3 x dx = int x/sin^3 x d(sin x) = int x d(-1/2 * 1/sin^2 x) =
= -1/2 * int x d(1/sin^2 x) = -1/2 * x * 1/sin^2 x + 1/2 * int 1/sin^2 x dx =
= -1/2 * x/sin^2 x - 1/2 * ctg x + C