Answer
$cos (-90^{\circ})=0$
Work Step by Step
1. What we know about -90 degree angle
$\theta=270^{\circ}$ (because it is a co-terminal angle and values of x and y will be the same)
$x=0$
$y=-1$
$r=1$
2. Substitute in:
$\cos\theta=\frac{x}{r}=\frac{0}{1}=0$
