Answer
$\sec (-540^{\circ}) =-1$
Work Step by Step
1. What we know about -540 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:
$\sec\theta=\frac{r}{x}=\frac{1}{-1}=-1$
