Answer
0
Work Step by Step
We can find the cosine of coterminal angle that lies between $0^{\circ}$ and $360^{\circ}$ for convenience.
$\cos(-90^{\circ})= \cos(-90^{\circ}+360^{\circ})=\cos 270^{\circ}$
The terminal side of $270^{\circ}$ angle passes through (0,-1). So $x=0,y=-1$ and $r=1$.
$\cos 270^{\circ}=\frac{x}{r}=\frac{0}{1}=0$
