Answer
-0.5
Work Step by Step
Since cosine is an even function, $\cos(-120^{\circ})=\cos(120^{\circ})$.
Using the unit circle, $120^{\circ}$ falls in the second quadrant, where sine is positive and both cosine and tangent are negative.
Therefore, by using the identity $\cos(180^{\circ}-x)=-\cos(x)$, $\cos(120^{\circ})=-\cos(180^{\circ}-120^{\circ})=-\cos(60^{\circ})$
Since $60^{\circ}$ is a special angle, $-\cos(60^{\circ})=-0.5$
