From Förberedande kurs i matematik 1

The easiest angle to find is \displaystyle v={\pi }/{3}\; in the first quadrant. When we draw the unit circle, we see that the angle which makes the same angle with the positive \displaystyle x -axis as \displaystyle v={\pi }/{3}\;, but is under the \displaystyle x -axis, also has a cosine value of \displaystyle {1}/{2}\; (same \displaystyle x -coordinate).

There are thus two angles, \displaystyle v={\pi }/{3}\; and \displaystyle v=2\pi -{\pi }/{3}\;={5\pi }/{3}\; which have their cosine value equal to \displaystyle \frac{1}{2}.