# 1.16: Coterminal Angles

Difficulty Level: At Grade Created by: CK-12
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Practice Coterminal Angles

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While playing a game with friends, you use a spinner that looks like this:

As you can see, the angle that the spinner makes with the horizontal is 60\begin{align*}60^\circ\end{align*}. Is it possible to represent the angle any other way?

### Coterminal Angles

Consider the angle 30\begin{align*}30^\circ\end{align*}, in standard position.

Now consider the angle 390\begin{align*}390^\circ\end{align*}. We can think of this angle as a full rotation (360)\begin{align*}(360^\circ)\end{align*}, plus an additional 30 degrees.

Notice that 390\begin{align*}390^\circ\end{align*} looks the same as 30\begin{align*}30^\circ\end{align*}. Formally, we say that the angles share the same terminal side. Therefore we call the angles co-terminal. Not only are these two angles co-terminal, but there are infinitely many angles that are co-terminal with these two angles. For example, if we rotate another 360\begin{align*}360^\circ\end{align*}, we get the angle 750\begin{align*}750^\circ\end{align*}. Or, if we create the angle in the negative direction (clockwise), we get the angle 330\begin{align*}-330^\circ\end{align*}. Because we can rotate in either direction, and we can rotate as many times as we want, we can continuously generate angles that are co-terminal with 30\begin{align*}30^\circ\end{align*}.

#### Identifying Co-Terminal Angles

For the following questions, determine if the angle is co-terminal with 45\begin{align*}45^\circ\end{align*}

1. 45\begin{align*}-45^\circ\end{align*}

No, it is not co-terminal with 45\begin{align*}45^\circ\end{align*}

2. 405\begin{align*}405^\circ\end{align*}

Yes, 405\begin{align*}405^\circ\end{align*} is co-terminal with 45\begin{align*}45^\circ\end{align*}.

3. 315\begin{align*}-315^\circ\end{align*}

Yes, 315\begin{align*}-315^\circ\end{align*} is co-terminal with 45\begin{align*}45^\circ\end{align*}.

### Examples

#### Example 1

Earlier, you were asked if it is possible to represent the angle any other way.

You can either think of 60\begin{align*}60^\circ\end{align*} as 420\begin{align*}420^\circ\end{align*} if you rotate all the way around the circle once and continue the rotation to where the spinner has stopped, or as 300\begin{align*}-300^\circ\end{align*} if you rotate clockwise around the circle instead of counterclockwise to where the spinner has stopped.

#### Example 2

Find a coterminal angle to 23\begin{align*}23^\circ\end{align*}

A coterminal angle would be an angle that is at the same terminal place as 23\begin{align*}23^\circ\end{align*} but has a different value. In this case, 337\begin{align*}-337^\circ\end{align*} is a coterminal angle.

#### Example 3

Find a coterminal angle to 90\begin{align*}-90^\circ\end{align*}

A coterminal angle would be an angle that is at the same terminal place as 90\begin{align*}-90^\circ\end{align*} but has a different value. In this case, 270\begin{align*}270^\circ\end{align*} is a coterminal angle.

#### Example 4

Find two coterminal angles to 70\begin{align*}70^\circ\end{align*} by rotating in the positive direction around the circle.

Rotating once around the circle gives a coterminal angle of 430\begin{align*}430^\circ\end{align*}. Rotating again around the circle gives a coterminal angle of 790\begin{align*}790^\circ\end{align*}.

### Review

1. Is 315\begin{align*}315^\circ\end{align*} co-terminal with 45\begin{align*}-45^\circ\end{align*}?
2. Is 90\begin{align*}90^\circ\end{align*} co-terminal with 90\begin{align*}-90^\circ\end{align*}?
3. Is 350\begin{align*}350^\circ\end{align*} co-terminal with 370\begin{align*}-370^\circ\end{align*}?
4. Is 15\begin{align*}15^\circ\end{align*} co-terminal with 1095\begin{align*}1095^\circ\end{align*}?
5. Is 85\begin{align*}85^\circ\end{align*} co-terminal with 1880\begin{align*}1880^\circ\end{align*}?

For each diagram, name the angle in 3 ways. At least one way should use negative degrees.

1. Name the angle of the 8 on a standard clock two different ways.
2. Name the angle of the 11 on a standard clock two different ways.
3. Name the angle of the 4 on a standard clock two different ways.
4. Explain how to determine whether or not two angles are co-terminal.
5. How many rotations is 4680\begin{align*}4680^\circ\end{align*}?

To see the Review answers, open this PDF file and look for section 1.16.

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### Vocabulary Language: English

TermDefinition
Coterminal Angles A set of coterminal angles are angles with the same terminal side but expressed differently, such as a different number of complete rotations around the unit circle or angles being expressed as positive versus negative angle measurements.

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