# 10.9: Arc Length

**At Grade**Created by: CK-12

**Practice**Arc Length

What if you wanted to find the "length" of the crust for an individual slice of pizza? A typical large pizza has a diameter of 14 inches and is cut into 8 or 10 pieces. If the "length" of the entire crust is the circumference of the pizza, find the "length" of the crust for one piece of pizza when the entire pizza is cut into a) 8 pieces or b) 10 pieces. After completing this Concept, you'll be able to answer these questions.

### Watch This

CK-12 Foundation: Chapter10ArcLengthA

### Guidance

One way to measure arcs is in degrees. This is called the “arc measure” or “degree measure.” Arcs can also be measured in length, as a portion of the circumference.
**
Arc length
**
is the length of an arc or a portion of a circle’s circumference. The arc length is directly related to the degree arc measure.

**
Arc Length Formula:
**
If
is the diameter or
is the radius, the length of
or
.

#### Example A

Find the length of . Leave your answer in terms of .

In the picture, the central angle that corresponds with is . This means that as well. So, think of the arc length as a portion of the circumference. There are in a circle, so would be of that . Therefore, the length of is of the circumference.

#### Example B

The arc length of and is the circumference. Find the radius of the circle.

If is the circumference, then the total circumference is . To find the radius, plug this into the circumference formula and solve for .

#### Example C

Find the measure of the central angle or .

Let’s plug in what we know to the Arc Length Formula.

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter10ArcLengthB

#### Concept Problem Revisited

In the picture below, the top piece of pizza is if it is cut into 8 pieces. Therefore, for of the pizza, one piece would have of crust. The bottom piece of pizza is if the pizza is cut into 10 pieces. For of the crust, one piece would have of crust.

### Vocabulary

**
Circumference
**
is the distance around a circle.

**is the length of an arc or a portion of a circle’s circumference.**

*Arc length*### Guided Practice

Find the arc length of in . Leave your answers in terms of .

1.

2.

3. An extra large pizza has a diameter of 20 inches and is cut into 12 pieces. Find the length of the crust for one piece of pizza.

**
Answers:
**

1. Use the Arc Length formula.

2. Use the Arc Length formula.

3. The entire length of the crust, or the circumference of the pizza, is . In of the pizza, one piece would have inches of crust.

### Practice

Find the arc length of in . Leave your answers in terms of .

Find (the radius) in . Leave your answer in terms of .

Find the central angle or in . Round any decimal answers to the nearest tenth.

- The Olympics symbol is five congruent circles arranged as shown below. Assume the top three circles are tangent to each other. Brad is tracing the entire symbol for a poster. How far will his pen point travel?

Mario’s Pizza Palace offers a stuffed crust pizza in three sizes (diameter length) for the indicated

prices:

The Little Cheese, 8 in, $7.00

The Big Cheese, 10 in, $9.00

The Cheese Monster, 12 in, $12.00

- What is the crust (in) to price ($) ratio for The Little Cheese?
- What is the crust (in) to price ($) ratio for The Little Cheese?
- What is the crust (in) to price ($) ratio for The Little Cheese?
- Michael thinks the cheesy crust is the best part of the pizza and wants to get the most crust for his money. Which pizza should he buy?

### Image Attributions

## Description

## Learning Objectives

Here you'll learn how to find the length of an arc.