int x * sin 4x dx = int x d(-1/4 * cos 4x) = -1/4 * int x d(cos 4x) =
= -1/4 * x * cos 4x + 1/4 * int cos 4x dx = -1/4 * x * cos 4x + 1/4 * 1/4 * sin 4x + C =
= -1/4 * x * cos 4x + 1/16 * sin 4x + C