I cos^2 3x dx (I=integral)

int cos²(3x) dx
= int (1+cos(6x)) / 2 dx
= int (1/2 + 1/2 cos(6x)) dx
= 1/2 x + 1/2 (1/6)sin(6x) + C
= 1/2 x + 1/12 sin(6x) + C