Answer
$sin(-420°)=-\frac{\sqrt 3}{2}$
$cos(-420°)=\frac{1}{2}$
$tan(-420°)=-\sqrt 3$
Work Step by Step
Let $θ'$ be the reference angle. We must have that $0°\leqθ'\lt360°$
$θ'=θ+n·360°$, where $n$ is an integer.
$θ'=-420°+2·360°=300°$
$sin(-420°)=sin~300°=-\frac{\sqrt 3}{2}$
$cos(-420°)=cos~300°=\frac{1}{2}$
$tan(-420°)=tan~300°=-\sqrt 3$
