#### Answer

$f(-x)=f(x)$

#### Work Step by Step

We take a look at the point where $x=2\pi$
At $x=2\pi$, $y=1$
Then we take a look at the point where $x=-2\pi$.
At $x=-2\pi$, $y=1$
Therefore, $f(-2\pi)=f(2\pi)=1$
We can take another case where $x=\pi$ and $x=-\pi$.
We find that $f(\pi)=f(-\pi)=-1$
In fact, as we take a close examination, the graph has an $Oy$ symmetry, which means every 2 points where $x=a$ and $x=-a$ has the same value of $y=f(a)$.
In other words, $f(-x)=f(x)$