## Trigonometry (11th Edition) Clone

$$\cos(0^\circ+\theta)=\cos\theta$$
$$A=\cos(0^\circ+\theta)$$ Here we need to use the cosine sum identity: $$\cos(A+B)=\cos A\cos B-\sin A\sin B$$ Applying that identity to $A$: $$A=\cos0^\circ\cos\theta-\sin0^\circ\sin\theta$$ $$A=1\times\cos\theta-0\times\sin\theta$$ $$A=\cos\theta$$