Let's make a graph of each side of our equation:
From the given graph, we can note that both graph are intersect each other when
x is multiple of Pi, that is,
[tex]x=\ldots-n\pi,\ldots,-3\pi,-2\pi,0,\pi,2\pi,\ldots..,n\pi[/tex]
which can be written as
[tex]x=n\times\pi\text{ }[/tex]
with n denoting an integer number.
Therefore, the equation is an identity when
[tex]x=n\times\pi\text{ with n}\in\text{ Integer}[/tex]
