9-8 Circles and Circumference
Warm Up
Problem of the Day
Lesson Presentation
Course 1
9-8 Circles and Circumference
Warm Up
The 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.
Course 1
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.
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9-8 Circles
Insert Lesson
Title Here
and Circumference
Vocabulary
circle
center
radius (radii)
diameter
circumference
pi
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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).
Radius
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Center
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.
Radius
Center
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.
L
Z
M
N
The circle is circle Z.
LM is a diameter.
ZL, ZM, and ZN are radii.
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9-8 Circles and Circumference
Check It Out: Example 1
Name the circle, a diameter, and three radii.
I
D
G
H
The circle is circle D.
IG is a diameter.
DI, DG, and DH are radii.
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9-8 Circles and Circumference
The distance around a circle is called the
circumference.
Circumference
Radius
Center
Diameter
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9-8 Circles and Circumference
The ratio of the circumference to the
diameter, C , is the same for any circle. This
d
ratio is represented by the Greek letter ,
which is read “pi.”
C
=
d
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9-8 Circles and Circumference
The decimal representation of pi starts with
3.14159265 . . . and goes on forever without
repeating. We estimate pi using either 3.14
22
or 7 .
The formula for the circumference of a circle is
C = d, or C = 2r.
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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
•
Write the formula.
8
C  24 ft
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Replace  with 3 and
with 8.
d
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
•
Write the formula.
14
C  42 yd
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Replace  with 3 and
d with 14.
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
Write the formula.
•
11
C  34.54 ft
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11 ft
Replace  with 3.14 and d with
11.
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 = ?
5 cm
C = 2r
C2
•
3.14
C  31.4 cm
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Write the formula.
•
5
Replace  with 3.14 and r
with 5.
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
21.98
3.14d
_______
 _______
3.14
3.14
7.00 cm  d
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Write the formula.
Replace C with 21.98 and 
with 3.14.
Divide both sides by 3.14.
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 = ?
9 ft
C = d
C  3.14
Write the formula.
•
9
C  28.26 ft
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Replace  with 3.14 and d with
9.
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 = ?
6 cm
C = 2r
C2
•
Write the formula.
3.14
•
C  37.68 cm
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6
Replace  with 3.14 and r
with 6.
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
18.84
3.14d
_______
 _______
3.14
3.14
6.00 cm  d
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Write the formula.
Replace C with 18.84 and 
with 3.14.
Divide both sides by 3.14.
9-8 Circles
Insert Lesson
and Circumference
Title Here
Lesson Quiz
Find the circumference of each circle. Use
3.14 for .
1.
8 in.
C = 25.12 in.
2.
3 in.
C = 18.84 in.
3. Find the circumference of a circle with
diameter of 20 feet. Use 3.14 for .
62.8 ft
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