Answer
- $\frac{1}{2}$
Work Step by Step
To find exact value of $\cos (-240)^{\circ}$, let's find its reference angle first. As $ (-240)^{\circ}$ terminates in quadrant II,
The reference angle = $ 240^{\circ} - 180^{\circ} $ = $60^{\circ}$
As $ (-240)^{\circ}$ terminates in quadrant II, its $\cos$ will be negative. Therefore by reference angle theorem-
$\cos (-240)^{\circ}$ = - $\cos 60^{\circ}$
= - $\frac{1}{2}$
