Question

Evaluate ∫ x cos 3x dx. A. 1/3 x sin 3x + 1/9 cos 3x + C B. 1/3 x sin 3x - 1/3 sin 3x + C C. 1/2 x2 + 1/18 sin2 3x + C D. 1/6 x2 sin 3x + C

Answer 1

I = ∫ cos (3x) dx
Let u = 3 x
du = 3 dx
du/3 = dx
I = (1/3) ∫ cos u du
I = (1/3) sin u + C
I = (1/3 sin (3x) + C 



Hope that helps!!!!