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?

#### Watch This

http://www.youtube.com/watch?v=nAJqXtzwpXQ James Sousa: Radian Measure

#### Guidance

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.

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Example A
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Convert into radians.

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Solution:
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Make sure the degree units cancel.

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Example B
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Convert radians into degrees.

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Solution:
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Often the ’s will cancel.

**
Example C
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Convert into radians.

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Solution:
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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.

**
Concept Problem Revisited
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Exactly radians describe a circular arc. This is because radiuses wrap around the circumference of any circle.

#### Vocabulary

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

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

#### Guided Practice

1. Convert into degrees.

2. Convert into radians.

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

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Answers:
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1.

2.

3.

#### Practice

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.