 Solution

The inequality is equivalent to $|\cos \theta |>1/\sqrt{2}$. The cosine exceeds $1/\sqrt{2}$ close to zero and $2\pi$, and is less than $-1/\sqrt{2}$ close to $\pi$. Within the specified interval this givesThe four equality angles are excluded because the inequality is strict; zero and $2\pi$ satisfy it and are included.