4.1 Radian and Degree Measure

What is an angle?

An angle is simply two rays that share a common endpoint. An angle is a vector meaning it has magnitude and direction. The starting position of a ray is refereed to as the initial side of the angle and the position remaining after rotation is refereed to as the terminal side. The common end point where the rays meet is the vertex.

Envisioning a coordinate system, an angle is in standard position if the initial side of the angle lies on the positive x-axis.

Positive angles result from counterclockwise rotation and negative angles result from clockwise rotation.








Angles are often labels with upper case or Greek letters. Angles are coterminal if they share a common initial and terminal side.

In this case, for all three angles created by the blue, red, and green lines, the resulting angles are coterminal.

What are we measuring when we measure an angle?
To measure an angle means to measure the amount of revolution. 

Since the circumference of a circle is 2πr, a central angle containing one full revolution counterclockwise creates an arc length s, equal to 2πr. This means that 2π radians correspond to 360° and π radians correspond to 180° and so on. 

If one circle (360°) is also 2π radians, there must be about 6.26 radians in one full circle. 

A circle can be divided and measured the following ways with the Unit Circle:

What are radians?
Radians are another way to measure angles besides degrees. Envision a central angle θ on a circle centered around the origin showing a vertex at the origin. A single radian is the amount of rotation needed to make the length of the intercepted arc s equal to the radius r.
If s = r then, θ =1 radian.









To determine arc length a proportion can be used.

Where...
Angle Measure: θ
Total Measure of One Circle: 360 (in degrees)  or 2π (in radians)
Arc Length: s (if degrees or radians are used for the total measure of one circle that same unit will remain for arc length)
Circumference: 2πr

Similarly, to convert from radians to degrees and vise versa a proportion is used.

Multiply the magnitude by... 

to go from degrees to radians and... 

to go from radians to degrees. The original unit will cancel out and the desired unit will remain.