Answer
$-1$
Work Step by Step
We know that $cos$ has a period of $2\pi$, hence first we try and find a value where the argument is between $-\pi$ and $/pi$. Therefore $cos(7\pi)=cos (7\pi-2\pi)=cos (5\pi)=cos (5\pi-2\pi)=cos (3\pi)=cos (3\pi-2\pi)=cos (\pi)=-1$
