9-8 Circles and Circumference

Course 1

Warm UpWarm Up

Lesson PresentationLesson Presentation

Problem of the DayProblem of the Day

Warm UpThe length and width of a rectangle are each multiplied by 5. Find how the perimeter and area of the rectangle change.

The perimeter is multiplied by 5, and the area is multiplied by 25.

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9-8 Circles and Circumference

Problem of the Day

When using a calculator to find the width of a rectangle whose length one knew, a student accidentally multiplied by 20 when she should have divided by 20. The answer displayed was 520. What is the correct width?1.3

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9-8 Circles and Circumference

Learn to identify the parts of a circle and to find the circumference of a circle.

Course 1

9-8 Circles and Circumference

Vocabulary

circlecenterradius (radii)diametercircumferencepi

Insert Lesson Title Here

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9-8 Circles and Circumference

A circle is the set of all points in a plane that are the same distance from a given point, called the center.

Center

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9-8 Circles and Circumference

A line segment with one endpoint at the center of the circle and the other endpoint on the circle is a radius (plural: radii).

CenterRadius

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9-8 Circles and Circumference

A diameter is a line segment that passes through the center of the circle and has both endpoints on the circle. The length of the diameter is twice the length of the radius.

CenterRadius

Diameter

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9-8 Circles and Circumference

Additional Example 1: Naming Parts of a Circle

Name the circle, a diameter, and three radii.

NThe circle is circle Z.

LM is a diameter.

ZL, ZM, and ZN are radii.

M

ZL

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9-8 Circles and Circumference

Check It Out: Example 1

Name the circle, a diameter, and three radii.

The circle is circle D.

IG is a diameter.

DI, DG, and DH are radii.

G

H

DI

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9-8 Circles and Circumference

The distance around a circle is called the circumference.

CenterRadius

Diameter

Circumference

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9-8 Circles and Circumference

The ratio of the circumference to the diameter, , is the same for any circle. This

ratio is represented by the Greek letter , which is read “pi.”

Cd

Cd

=

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9-8 Circles and Circumference

The formula for the circumference of a circle is C = d, or C = 2r.

The decimal representation of pi starts with 3.14159265 . . . and goes on forever without repeating. We estimate pi using either 3.14

or .22 7

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9-8 Circles and Circumference

Additional Example 2: Application

A skydiver is laying out a circular target for his next jump. Estimate the circumference of the target by rounding to 3.

C = d

C 3 • 8

C 24 ft

Write the formula.

Replace with 3 and d with 8.

8 ft

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9-8 Circles and Circumference

Check It Out: Example 2

A second skydiver is laying out a circular target for his next jump. Estimate the circumference of the target by rounding to 3.

C = d

C 3 • 14

C 42 yd

Write the formula.

Replace with 3 and d with 14.

14 yd

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9-8 Circles and Circumference

Additional Example 3A: Using the Formula for the Circumference of a Circle

Find the missing value to the nearest hundredth. Use 3.14 for pi.

d = 11 ft; C = ?

C = d

C 3.14 • 11

C 34.54 ft

Write the formula.

Replace with 3.14 and d with 11.

11 ft

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9-8 Circles and Circumference

Additional Example 3B: Using the Formula for the Circumference of a Circle

Find each missing value to the nearest hundredth. Use 3.14 for pi.

r = 5 cm; C = ?

C = 2r

C 2 • 3.14 • 5

C 31.4 cm

Write the formula.

Replace with 3.14 and r with 5.

5 cm

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9-8 Circles and Circumference

Additional Example 3C: Using the Formula for the Circumference of a Circle

Find each missing value to the nearest hundredth. Use 3.14 for pi.

C = 21.98 cm; d = ?

C = d

21.98 3.14d

7.00 cm d

Write the formula.

Replace C with 21.98 and with 3.14.

21.98 3.14d_______ _______

3.14 3.14 Divide both sides by 3.14.

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9-8 Circles and Circumference

Check It Out: Example 3A

Find the missing value to the nearest hundredth. Use 3.14 for pi.

d = 9 ft; C = ?

C = d

C 3.14 • 9

C 28.26 ft

Write the formula.

Replace with 3.14 and d with 9.

9 ft

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9-8 Circles and Circumference

Check It Out: Example 3B

Find each missing value to the nearest hundredth. Use 3.14 for pi.

r = 6 cm; C = ?

C = 2r

C 2 • 3.14 • 6

C 37.68 cm

Write the formula.

Replace with 3.14 and r with 6.

6 cm

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9-8 Circles and Circumference

Check It Out: Example 3C

Find each missing value to the nearest hundredth. Use 3.14 for pi.

C = 18.84 cm; d = ?

C = d

18.84 3.14d

6.00 cm d

Write the formula.

Replace C with 18.84 and with 3.14.

18.84 3.14d_______ _______

3.14 3.14 Divide both sides by 3.14.

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9-8 Circles and Circumference

Lesson Quiz

Find the circumference of each circle. Use 3.14 for .

1. 2.

3. Find the circumference of a circle with

diameter of 20 feet. Use 3.14 for .

C = 25.12 in.

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C = 18.84 in.

8 in.

62.8 ft

3 in.

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9-8 Circles and Circumference