Answer:
(B) π/12 + π/6 k
Explanation:
Points of inflection are where f"(x) = 0 and changes signs.
f(x) = cos²(3x)
f(x) = (cos(3x))²
f'(x) = 2 (cos(3x))¹ × -sin(3x) × 3
f'(x) = -6 sin(3x) cos(3x)
Using double angle formula:
f'(x) = -3 sin(6x)
f"(x) = -3 cos(6x) × 6
f"(x) = -18 cos(6x)
0 = -18 cos(6x)
0 = cos(6x)
6x = π/2 + 2πk or 6x = 3π/2 + 2πk
We can simplify this to:
6x = π/2 + πk
x = π/12 + π/6 k