soal integral 1. ∫sin x cos 4x dx

∫ cos 4x sin x dx = ∫ sin (4x+x) - sin (4x-x) dx = ∫ sin 5x - sin 3x dx
= - 1/5 cos 5x + 1/3 cos 3x + c
= 1/15 ( - 3 cos 5x + 5 cos x )+ c