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Conversion between Degrees and Radians

Convert between radians and degrees

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Angles in Radians and Degrees

Most people are familiar with measuring angles in degrees. It is easy to picture angles like or  and the fact that  makes up an entire circle. Over 2000 years ago the Babylonians used a base 60 number system and divided up a circle into 360 equal parts. This became the standard and it is how most people think of angles today. 

However, there are many units with which to measure angles. For example, the gradian was invented along with the metric system and it divides a circle into 400 equal parts. The sizes of these different units are very arbitrary. 

A radian is a unit of measuring angles that is based on the properties of circles. This makes it more meaningful than gradians or degrees. How many radians make up a circle? 

Radians and Degrees

A radian is defined to be the central angle where the subtended arc length is the same length as the radius. 

Another way to think about radians is through the circumference of a circle. The circumference of a circle with radius  is . Just over six radii (exactly  radii) would stretch around any circle. 

To define a radian in terms of degrees, equate a circle measured in degrees to a circle measured in radians. 

, so

Alternatively; , so

The conversion factor to convert degrees to radians is:  

The conversion factor to convert radians to degrees is: 

If an angle has no units, it is assumed to be in radians. 

If you were to convert  into radians, you would multiply  by the correct conversion factor. You would get:

 

You can check your work by making sure the degree units cancel. 

If you were to convert  radians into degrees, you would multiply  by the correct conversion factor. You would get 

Often the ’s will cancel. 

    

 

Examples

Example 1

Earlier, you were asked how many radians make up a circle. Exactly  radians describe a circular arc. This is because  radii wrap around the circumference of any circle. 

Example 2

Convert  into radians. 

Don’t be fooled just because this has . This number is about

It is very unusual to ever have a  term, but it can happen. 

Example 3

Convert  into degrees. 

Example 4

Convert  into radians. 

INSERT 4

Example 5

Draw a  angle by first drawing a  angle, halving it and halving the result. 

Review

Find the radian measure of each angle.

1. 

2.

3.

4.

5.

6. 

7.

Find the degree measure of each angle.

8. 

9. 

10. 

11. 

12.

13.

14. 3

15. Explain why if you are given an angle in degrees and you multiply it by  you will get the same angle in radians.

Review (Answers)

To see the Review answers, open this PDF file and look for section 4.1. 

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Vocabulary

subtended arc

A subtended arc is the part of the circle in between the two rays that make the central angle.

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