Derivative of $$$1 - \cos{\left(x \right)}$$$

The derivative of a sum/difference is the sum/difference of derivatives:

$${\color{red}\left(\frac{d}{dx} \left(1 - \cos{\left(x \right)}\right)\right)} = {\color{red}\left(\frac{d}{dx} \left(1\right) - \frac{d}{dx} \left(\cos{\left(x \right)}\right)\right)}$$

The derivative of the cosine is $$$\frac{d}{dx} \left(\cos{\left(x \right)}\right) = - \sin{\left(x \right)}$$$:

$$- {\color{red}\left(\frac{d}{dx} \left(\cos{\left(x \right)}\right)\right)} + \frac{d}{dx} \left(1\right) = - {\color{red}\left(- \sin{\left(x \right)}\right)} + \frac{d}{dx} \left(1\right)$$

The derivative of a constant is $$$0$$$:

$$\sin{\left(x \right)} + {\color{red}\left(\frac{d}{dx} \left(1\right)\right)} = \sin{\left(x \right)} + {\color{red}\left(0\right)}$$

Thus, $$$\frac{d}{dx} \left(1 - \cos{\left(x \right)}\right) = \sin{\left(x \right)}$$$.