Using the fact that $\sin (A+B)=\sin A \cos B+\cos A \sin B$ and the differentiation,obtain the sum formula for cosines.

Given the identity: $\sin (A+B)=\sin A \cos B+\cos A \sin B$
Differentiating both sides with respect to $x$,we apply the product rule and chain rule:
$\frac{d}{d x}[\sin (A+B)]=\frac{d}{d x}(\sin A \cos B)+\frac{d}{d x}(\cos A \sin B)$
$\cos (A+B) \cdot \frac{d}{d x}(A+B) = \cos B \cdot \frac{d}{d x}(\sin A) + \sin A \cdot \frac{d}{d x}(\cos B) + \sin B \cdot \frac{d}{d x}(\cos A) + \cos A \cdot \frac{d}{d x}(\sin B)$
Using the chain rule,$\frac{d}{dx}(\sin A) = \cos A \frac{dA}{dx}$,$\frac{d}{dx}(\cos A) = -\sin A \frac{dA}{dx}$,etc.:
$\cos (A+B) \left( \frac{dA}{dx} + \frac{dB}{dx} \right) = \cos B (\cos A \frac{dA}{dx}) + \sin A (-\sin B \frac{dB}{dx}) + \sin B (-\sin A \frac{dA}{dx}) + \cos A (\cos B \frac{dB}{dx})$
$\cos (A+B) \left( \frac{dA}{dx} + \frac{dB}{dx} \right) = \cos A \cos B \frac{dA}{dx} - \sin A \sin B \frac{dB}{dx} - \sin A \sin B \frac{dA}{dx} + \cos A \cos B \frac{dB}{dx}$
$\cos (A+B) \left( \frac{dA}{dx} + \frac{dB}{dx} \right) = \cos A \cos B \left( \frac{dA}{dx} + \frac{dB}{dx} \right) - \sin A \sin B \left( \frac{dA}{dx} + \frac{dB}{dx} \right)$
$\cos (A+B) \left( \frac{dA}{dx} + \frac{dB}{dx} \right) = (\cos A \cos B - \sin A \sin B) \left( \frac{dA}{dx} + \frac{dB}{dx} \right)$
Canceling the common term $\left( \frac{dA}{dx} + \frac{dB}{dx} \right)$,we get:
$\cos (A+B) = \cos A \cos B - \sin A \sin B$