I recall that $\cos 120^\circ$ is one of the special values of the cosine function that can be found using the unit circle. I also remember that $\cos 120^\circ = -\frac{1}{2}$, since it is the x-coordinate of the point on the unit circle that has an angle of 120 degrees with the positive x-axis. Therefore, I can write my answer as $\cos 120^\circ = -\frac{1}{2}$. 
Compute $\cos 120^\circ$.