Circumference of Circles

8/14/2019 Circumference of Circles

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9 Circumference of Circles

Course 2

Warm Up

Problem of the Day

Lesson Presentation


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Learnto find the circumference of a

circle.

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Vocabularycircumference

Insert Lesson Title Here

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Course 2


Cd

The distance around a circle is called circumference.For every circle, the ratio of circumference C todiameter dis the same. This ratio, , is

represented by the Greek letter

, calledpi. Pi isapproximately equal to 3.14 or . By solving the227

equation = for C, you getthe formula forcircumference. the formula for circumference.

C =

d, or C = 2

r.

Cd


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Course 2


Radius

Diameter

Circumference

C = d,

orC = 2r

CIRCUMFERENCE OF A CIRCLE

The

circumference

Cof a circle is times the

diameter d, or

2times the

radius r.


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Find the circumference of the circle to the

nearest tenth. Use 3.14 for .

Additional Example 3A: Finding the Circumference

of a Circle

12 in.C = d

C 3.14 12

C 37.68

You know the diameter.

Substitute for and d.

Multiply.

The circumference of the circle is about 37.7 in.

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Additional Example 3B: Finding the Circumference

of a Circle

18 cmC = 2r

C 2 3.14 18

C 113.04

You know the radius.

Substitute for and r.

Multiply.

The circumference of the circle is about 113.0 cm.

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Check It Out: Example 3A

7 in.C = d

C 3.14 7

C 21.98

You know the diameter.

Substitute for and d.

Multiply.

The circumference of the circle is about 22.0 in.

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Check It Out: Example 3B

11 cmC = 2r

C 2 3.14 11

C 69.08

You know the radius.

Substitute for and r.

Multiply.

The circumference of the circle is about 69.1 cm.

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The diameter of a circular pond is 42 m. Whatis its circumference? Use

Additional Example 4: Application

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227

for .

C= d You know the diameter.

C 227

42 Substitute 227 forand 42 for d.

C 227

421

C 227

421

6

1C132

The circumference of the pond is about 132 m.

Write 42 as a fraction.

Simplify.

Multiply.


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The diameter of a circular spa is 14 m. Whatis its circumference? Use

Check It Out: Example 4

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227

for .

C= d You know the diameter.

C 227

14 Substitute 227 forand 14 for d.

C 227

141

C 227

141

2

1C44

The circumference of the spa is about 44 m.

Write 14 as a fraction.

Simplify.

Multiply.


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Lesson Quiz

Find the circumference.

1.What is the circumference of a circle with a

a radius of 8 inches? Use 3.14 for .

2.If a guardrail is to be placed around the edgeof a circular swimming pool with a diameter of21 ft, how many feet of railing will be needed?

Insert Lesson Title Here

50.24 in.

66 ft

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227

Use for .

Circumference of Circles

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