Answer
$$\tan \left(x\right)$$
Work Step by Step
Simplifying the expression using the sum and difference formulas, we find:
$$\frac{\cos \left(x\right)\sin \left(\pi \right)+\cos \left(\pi \right)\sin \left(x\right)}{\cos \left(x+\pi \right)} \\ \frac{-\sin \left(x\right)}{-\cos \left(x\right)} \\ \tan \left(x\right)$$
