## Trigonometry (11th Edition) Clone

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