Derivada de $$$\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}$$$ con respecto a $$$x$$$

La derivada de una suma/diferencia es la suma/diferencia de las derivadas:

$${\color{red}\left(\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right)\right)} = {\color{red}\left(\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right) - \frac{d}{dx} \left(\sin{\left(x \right)} \cos{\left(a \right)}\right)\right)}$$

Aplica la regla del factor constante $$$\frac{d}{dx} \left(c f{\left(x \right)}\right) = c \frac{d}{dx} \left(f{\left(x \right)}\right)$$$ con $$$c = \cos{\left(a \right)}$$$ y $$$f{\left(x \right)} = \sin{\left(x \right)}$$$:

$$- {\color{red}\left(\frac{d}{dx} \left(\sin{\left(x \right)} \cos{\left(a \right)}\right)\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right) = - {\color{red}\left(\cos{\left(a \right)} \frac{d}{dx} \left(\sin{\left(x \right)}\right)\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right)$$

La derivada del seno es $$$\frac{d}{dx} \left(\sin{\left(x \right)}\right) = \cos{\left(x \right)}$$$:

$$- \cos{\left(a \right)} {\color{red}\left(\frac{d}{dx} \left(\sin{\left(x \right)}\right)\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right) = - \cos{\left(a \right)} {\color{red}\left(\cos{\left(x \right)}\right)} + \frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right)$$

Aplica la regla del factor constante $$$\frac{d}{dx} \left(c f{\left(x \right)}\right) = c \frac{d}{dx} \left(f{\left(x \right)}\right)$$$ con $$$c = \sin{\left(a \right)}$$$ y $$$f{\left(x \right)} = \cos{\left(x \right)}$$$:

$$- \cos{\left(a \right)} \cos{\left(x \right)} + {\color{red}\left(\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)}\right)\right)} = - \cos{\left(a \right)} \cos{\left(x \right)} + {\color{red}\left(\sin{\left(a \right)} \frac{d}{dx} \left(\cos{\left(x \right)}\right)\right)}$$

La derivada del coseno es $$$\frac{d}{dx} \left(\cos{\left(x \right)}\right) = - \sin{\left(x \right)}$$$:

$$\sin{\left(a \right)} {\color{red}\left(\frac{d}{dx} \left(\cos{\left(x \right)}\right)\right)} - \cos{\left(a \right)} \cos{\left(x \right)} = \sin{\left(a \right)} {\color{red}\left(- \sin{\left(x \right)}\right)} - \cos{\left(a \right)} \cos{\left(x \right)}$$

Simplificar:

$$- \sin{\left(a \right)} \sin{\left(x \right)} - \cos{\left(a \right)} \cos{\left(x \right)} = - \cos{\left(a - x \right)}$$

Por lo tanto, $$$\frac{d}{dx} \left(\sin{\left(a \right)} \cos{\left(x \right)} - \sin{\left(x \right)} \cos{\left(a \right)}\right) = - \cos{\left(a - x \right)}$$$.