Given a coordinate system,
an angle (a radian on the unit circle)
always starts from the positive -axis, that is,
from the point .
We have the radian
if the angle with the terminal side
is constructed *counterclockwise*.

We have
if it is *clockwise*.

In this setting of coordinate system,
the cosine and the sine functions of are defined by

That is, the point on the unit circle with the radian
is given by
The other trigonometric functions of
are then determined by (1-2) and (1-3).

Department of Mathematics

*Last modified: 2005-09-29*