## Trigonometry (11th Edition) Clone

$$\cos(60^\circ+\theta)=\frac{\cos\theta-\sqrt3\sin\theta}{2}$$
$$X=\cos(60^\circ+\theta)$$ To expand the formula, cosine sum identity would be used: $$\cos(A+B)=\cos A\cos B-\sin A\sin B$$ That means $$X=\cos60^\circ\cos\theta-\sin60^\circ\sin\theta$$ $$X=\frac{1}{2}\cos\theta-\frac{\sqrt3}{2}\sin\theta$$ $$X=\frac{\cos\theta-\sqrt3\sin\theta}{2}$$ Therefore, $$\cos(60^\circ+\theta)=\frac{\cos\theta-\sqrt3\sin\theta}{2}$$