Answer
$$\theta=20^\circ$$
Work Step by Step
$$\sin\theta=\cos(2\theta+30^\circ)\hspace{1cm}(1)$$
- Identity of the cofunction:
$$\sin\theta=\cos(90^\circ-\theta)$$
Replace into $(1)$:
$$\cos(90^\circ-\theta)=\cos(2\theta+30^\circ)$$
$$90^\circ-\theta=2\theta+30^\circ$$
$$3\theta=60^\circ$$
$$\theta=20^\circ$$
