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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.
Course 2
9 Circumference of Circles
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Vocabularycircumference
Insert Lesson Title Here
Course 2
9 Circumference of Circles
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Course 2
9 Circumference of Circles
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
9 Circumference of Circles
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.
Course 2
9 Circumference of Circles
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Find the circumference of the circle to the
nearest tenth. Use 3.14 for .
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.
Course 2
9 Circumference of Circles
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Find the circumference of the circle to the
nearest tenth. Use 3.14 for .
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.
Course 2
9 Circumference of Circles
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Find the circumference of the circle to the
nearest tenth. Use 3.14 for .
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.
Course 2
9 Circumference of Circles
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The diameter of a circular pond is 42 m. Whatis its circumference? Use
Additional Example 4: Application
Course 2
9 Circumference of Circles
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
Course 2
9 Circumference of Circles
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
Course 2
9 Circumference of Circles
227
Use for .