In order to get an angle between 0 and 2, we subtract 2 from 72, which also leaves the cosine value unchanged
cos27=cos27−2=cos23.
When we draw a line which makes an angle 32 with the positive x-axis, we get the negative y-axis and we see that this line cuts the unit circle at the point (0,-1). The x-coordinate of the intersection point is thus
0 and hence cos(72)=cos(32)=0.
2
27
=cos23
