#### Radian Measure of Angles

We start this section off by recalling that an angle AOB (denoted
AOB) consists of two rays R_{1}
and R_{2} with a common vertex O. An angle is often thought of as a
rotation of the ray R_{1} onto R_{2} where R_{1} is the
initial side and R_{2} is called the terminal side of the angle. A
counterclockwise rotation is positive and a clockwise rotation is negative. The
amount of rotation about the vertex is known as the measure of an angle. When
it is rotated counterclockwise until it coincides with itself it is said to
measure 360 degrees which is one revolution.

If a unit circle is drawn with the vertex of an angle at its center, then the measure of this angle in RADIANS is the length of the arc that subtends the angle. Take a look at the diagram to see what we mean.

As we saw in the circles section, the circumference of the circle of radius 1 is 2. Therefore one complete revolution has a measure of 2 radians. Also note that a straight angle has a measure radians, and a right angle has a measure /2 radians.

So what is the relationship between degrees and radians? It's easy to figure
out. We know that a straight angle is 180 degrees and it has a measure of
radians. Therefore
^{o} = radians.^{o},^{o} =
(/180) rad.

When an angle is drawn in the xy-plane with the vertex at the origin and its
initial side on the positive x-axis, then it is said to be in standard
position. If two angles in standard position are coinciding, then there are
coterminal. Here is a diagram to help
visualize this. To find a coterminal angle given another angle, all we must do
is add a multiple of 360^{o} if the angle is measured in degrees or add
a multiple of 2 if the
angle is measured in radians. If the multiple is positive, then it is a
positive coterminal angle and if it is a negative multiple then it is a
negative coterminal angle.

We can figure out the arc length of a circle if we know the radius and we know
the measure of the angle which the arc subtends. For instance, if the angle
measures radians, the arc length s is

- Note:
- This is how we came up with the formula. Since the fraction of the circle
contained by the arc is /2 and the circumference of
the circle is given by 2r, we get the arc length

s = (/2)(2r) = r.

A similar thing can be done if we know the area of a circle. Instead of finding the length of the arc, we find the area of the sector of the circle. This is given by the formula

^{2}) = r

^{2}/2.