Graphing Calculator Math Worksheets Algebara Solver Chart Maker
 A+ A− B

## What is the deal with radians anyway?

Answer: Most of you are used to thinking of a circle in terms of degrees: 360° is the whole circle. 180° is half the circle etc... Well, radian measure is just a different way of talking about the circle. Radian measure is just different unit of measure.

Just as we can measure a football field in yards or feet--we can measure a circle in degrees (like the good old days) or in radians (welcome to the big leagues!)

Think about what the word radian sounds like...well, it sounds like 'radius', right? It turns out that a radian has a close relationship to the radius of a circle

## So what is a radian then?

Definition of radian : a radian is the measure of an angle that, when drawn a central angle of a circle, intercepts an arc whose length is equal to the length of the radius of the circle.

### Demonstration of Radian of Circle

The video below is an animation of 1 radian. Notice how the length of 1 radius stretches out to a portion of the circle. That portion is 1 radian of the circle.

There is a simple formula to convert radians to degrees. 1Π radian = 180. Therefore you can easily convert from one unit of measure to the other

The general formula for converting from degrees to radians is to simply multiply the number of degree by Π /180°
Example 1:

$200 ^{\circ} \cdot \frac{\pi }{ 180 ^{\circ}} \\ = \frac{10\pi}{ 9} \text{ radians } = \text{3.49 radians }$
Example 2:

$120 ^{\circ} \cdot \frac{\pi }{ 180} = \frac{2\pi}{ 3}\text{ radians }= \text{2.09 radians }$

The general formula for converting from radians to degrees to simply multiply the number of degree by 180°/(Π)
Example 1:

Convert $$\frac{4}{9} \pi \text{ radians}$$ to degrees $\frac{4\pi}{9} \cdot \frac{180 ^{\circ} }{\pi} \\ = \frac{4\pi \cdot 180 ^{\circ} \cdot}{9\pi} =\frac{720 ^{\circ} \pi \cdot}{9\pi} \\ \frac{720 ^{\circ} \cancel{ \color{Red} \pi} \cdot}{9\cancel{ \color{Red} \pi}} \\ = 80 ^{\circ}$
Example 2

Convert 1.4 radians into degrees: $1.4 \cdot \frac{180 ^{\circ} }{\pi} \\ =\frac{252 ^{\circ} }{\pi} \approx 80.2^{\circ}$

## So what's the deal with '$$\pi$$ radians' vs 'radians' ?

Answer: Let's rephrase the question as follows :
" Is there any difference between 5 radians and $$5\pi \text{ radians }$$? "

#### Practice Converting Degrees between Radians

Problem 1) What is the radian measure of 60°?

Problem 2) What is the degree measure of an arc whose measure is $$\frac{2\pi}{3} \text{ radians}$$?
Problem 3) What is the degree measure of an arc whose measure is $$\frac{7\pi}{12} \text{ radians}$$?