Let | cos θ cos (60-θ) cos (60+θ)| ≤ 1/8, θ ∈[0,2 π]. Then, the sum of all θ ∈[0,2 π], where cos 3 θ attains its maximum value, is:

Let $\displaystyle |\cos \theta \cos (60-\theta) \cos (60+\theta)| \leq \frac{1}{8}, \theta \in[0,2 \pi]$. Then, the sum of all $\displaystyle \theta \in[0,2 \pi]$, where $\displaystyle \cos 3 \theta$ attains its maximum value, is :