Aus Online Mathematik Brückenkurs 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 x-axis as \displaystyle v=\pi/3, but is under the x-axis, also has a cosine value of 1/2 (same 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 1/2.