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2.2: Conversion between Degrees and Radians

Difficulty Level: At Grade Created by: CK-12
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Practice Conversion between Degrees and Radians
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You are hard at work in the school science lab when your teacher asks you to turn a knob on a detector you are using 75 degrees. Unfortunately, you have been working in radians for a while, and so you're having trouble remembering how far to turn the knob. Is there a way to translate the instructions in degrees to radians?

Read this Concept, and at the conclusion you'll be able to accomplish this task and turn the knob the appropriate amount.

Watch This

James Sousa Example: Converting Angles in Degree Measure to Radian Measure

Guidance

Since degrees and radians are different ways of measuring the distance moved around the circumference of a circle, it is reasonable to suppose that there is a conversion formula between these two units. This formula works for all degrees and radians. Remember that: π radians=180. If you divide both sides of this equation by π, you will have the conversion formula:

radians×180π=degrees

If we have a degree measure and wish to convert it to radians, then manipulating the equation above gives:

degrees×π180=radians

Example A

Convert 11π3 to degree measure.

From the last section, you should recognize that this angle is a multiple of π3 (or 60 degrees), so there are 11, π3's in this angle, π3×11=60×11=660.

Here is what it would look like using the formula:

radians×180π=degrees

Example B

Convert 120 to radian measure. Leave the answer in terms of π.

degrees×π180120×π180=radians=120π180

and reducing to lowest terms gives us 2π3

You could also have noticed that 120 is 2×60. Since 60 is π3 radians, then 120 is 2, π3’s, or 2π3. Make it negative and you have the answer, 2π3.

Example C

Express 11π12 radians terms of degrees.

radians×180π=degrees

Note: Sometimes students have trouble remembering if it is 180π or π180. It might be helpful to remember that radian measure is almost always expressed in terms of π. If you want to convert from radians to degrees, you want the π to cancel out when you multiply, so it must be in the denominator.

Vocabulary

Radian: A radian (abbreviated rad) is the angle created by bending the radius length around the arc of a circle.

Degree: A degree is a unit for measuring angles in a circle. There are 360 of them in a circle.

Guided Practice

1. Convert the following degree measures to radians. All answers should be in terms of π.

240, 270, 315, 210, 120

2.Convert the following degree measures to radians. All answers should be in terms of π.

15, 450, 72, 720, 330

3. Convert the following radian measures to degrees

π2, 11π5, 2π3, 5π, 7π2

Solutions:

1. 4π3, 3π2, 7π4, 7π6, 2π3

2. π12, 5π2, \begin{align*}\frac{\pi}{5}\end{align*}, \begin{align*}4 \pi\end{align*}, \begin{align*}\frac{11\pi}{6}\end{align*}

3. \begin{align*}90^\circ\end{align*}, \begin{align*}396^\circ\end{align*}, \begin{align*}120^\circ\end{align*}, \begin{align*}540^\circ\end{align*}, \begin{align*}630^\circ\end{align*}

Concept Problem Solution

Since you now know that the conversion for a measurement in degrees to radians is

\begin{align*}\text{degrees} \times \frac{\pi}{180}=\text{radians}\end{align*}

you can find the solution to convert \begin{align*}75^\circ\end{align*} to radians:

\begin{align*}75^\circ \times \frac{\pi}{180}= \frac{75\pi}{180} = \frac{5\pi}{12}\end{align*}

Practice

Convert the following degree measures to radians. All answers should be in terms of \begin{align*}\pi\end{align*}.

  1. \begin{align*}90^\circ\end{align*}
  2. \begin{align*}360^\circ\end{align*}
  3. \begin{align*}50^\circ\end{align*}
  4. \begin{align*}110^\circ\end{align*}
  5. \begin{align*}495^\circ\end{align*}
  6. \begin{align*}-85^\circ\end{align*}
  7. \begin{align*}-120^\circ\end{align*}

Convert the following radian measures to degrees.

  1. \begin{align*}\frac{5\pi}{12}\end{align*}
  2. \begin{align*}\frac{3\pi}{5}\end{align*}
  3. \begin{align*}\frac{8\pi}{15}\end{align*}
  4. \begin{align*}\frac{7\pi}{10}\end{align*}
  5. \begin{align*}\frac{5\pi}{2}\end{align*}
  6. \begin{align*}3\pi\end{align*}
  7. \begin{align*}\frac{7\pi}{2}\end{align*}
  8. Why do you think there are two different ways to measure angles? When do you think it might be more convenient to use radians than degrees?

Vocabulary

Degree

A degree is a unit for measuring angles in a circle. There are 360 degrees in a circle.

radian

A radian is a unit of angle that is equal to the angle created at the center of a circle whose arc is equal in length to the radius.

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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Sep 26, 2012
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