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
cos72=cos32=0