Answer
All real numbers $x$ such that $x \ne 2\pi k $, where $k$ is an integer
Work Step by Step
The denominator must not be equal to zero:
$1-\cos x \ne 0$
$\cos x \ne 1$
$x \ne 0, 2\pi, -2\pi, 4\pi, -4\pi ...$
The domain is all real numbers $x$ such that $x \ne 2\pi k $, where $k$ is an integer
