**Radian.**
We will consider a circle of radius ,
and call it the *unit circle*.
Then an arc length of the unit circle is called
a *radian*, typically denoted by .
Recall that the circumference of the unit circle is .
For example,
, and
are equal to
,
and
.

**Special right triangles.**
The right triangle with adjacent side
and opposite side gives the hypotenuse
and the angle
.

The right triangle with opposite side and hypotenuse gives the adjacent side and the angle .

Department of Mathematics