From Förberedande kurs i matematik 1

On a unit circle, the angle \displaystyle {3\pi }/{2}\; corresponds to the point \displaystyle \left( 0 \right.,\left. -1 \right), 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}\;.