From Förberedande kurs i matematik 1

On the unit circle, the angle \displaystyle 3\pi/2 corresponds to the point (0,-1), and the angle in the interval from \displaystyle 0 to \displaystyle \pi which has the same cosine value as \displaystyle 3\pi/2, i.e. the x-coordinate \displaystyle 0, is the angle \displaystyle v = \pi/2\,.