Answer
$\cos(-900)^{\circ}=-1$
Work Step by Step
1. What we know about -900 degree angle
$\theta=180^{\circ}$ (because it is a co-terminal angle and values of x and y will be the same)
$x=-1$
$y=0$
$r=1$
2. Substitute in:
$\cos\theta=\frac{x}{r}=\frac{-1}{1}=-1$
