# How do you calculate cos((3pi)/2)?

If we consider the goniometric circle, that is a circle centered in the origin of axes with radius $1$, a point that lies on it has the coordinates: $\left(\cos \alpha , \sin \alpha\right)$, where $\alpha$ is the angle (in radians) that the radius makes with the positive real axis Ox.
Since $\alpha = \frac{3}{2} \pi$, than the point has the coordinates $\left(0 , - 1\right)$, and so $\cos \left(\frac{3}{2} \pi\right) = 0$.