Mathematics - Trigonometric Ratios & Identities Question with Solution | TestHub

Using $\cos 3\theta +\sin 3\theta =(\cos \theta -\sin \theta )(1+4\sin \theta \cos \theta )$, the full expression simplifies to $4(\cos \theta -\sin \theta )=4\sqrt{2}\cos (\theta +$ $\pi /4)$. It is positive on $(2n\pi -3\pi /4,2n\pi +\pi /4)$, so A and its subset B qualify.