Answer
See the full explanation below.
Work Step by Step
Let us consider the left side of the given expression:
$\cos \left( \pi -x \right)$
By using the identity of trigonometry, $\cos \,\left( \alpha -\beta \right)=\cos \,\alpha \,\cos \beta +\sin \,\alpha \,\sin \,\beta $, the above expression can be further simplified as:
$\begin{align}
& \cos \left( \pi -x \right)=\cos \,\pi \,\cos \,x+\sin \,\pi \,\sin \,x \\
& =-1\times \cos \,x+0\times \sin \,x \\
& =-\cos \,x+0 \\
& =-\cos \,x
\end{align}$
Hence, the left side of the expression is equal to the right side, which is $\cos \left( \pi -x \right)=-\cos \,x$.
