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Circumference
Vocabulary
• Center – the point in the middle of the
circle
• Circumference – the distance around a
circle.
• Diameter – distance across a circle
through its center.
• Radius – the distance from the center to
any point on the circle.
Hula Hoops
Size
Radius
Diameter Circumference
Child
14 in.
28 in.
88 in.
Adult
20 in.
40 in.
126 in.
The table shows the approximate measurements
of two sizes of hula hoops.
1.
Describe the relationship between the diameter and
radius of each hula hoop.
The diameter is twice the radius.
2.
Describe the relationship between the circumference
and diameter of each hula hoop.
The circumference is about three times the diameter.
Radius and Diameter
• The diameter of a circle, d, is twice its
radius.
d = 2r
• The radius, r, of a circle is half of its
diameter.
d
r=
2
Radius & Diameter Examples
1. The diameter of a circle is 14 inches.
Find the radius. r  d
The radius of the circle is
7 inches.
2
14
r
2
14 in.
r 7
2. The radius of a circle is 8 feet. Find the
diameter.
d  2r
d  2(8)
d  16
The diameter of the
circle is 16 feet.
8 ft.
Practice Problems
• Find the radius or diameter of each circle
with the given dimension.
1. d = 23 cm
The radius is 11.5
centimeters.
3. d = 16 yd
The radius is 8 yards.
2. r = 3 in.
The diameter is 6 inches.
4. r = 5.2 m
The diameter is 10.4
meters.
Circumference
• The circumference of a circle is equal to π times
its diameter or π times twice its radius.
C = π(d) or C = 2(π)(r)
Example: Find the circumference of a circle with a
diameter of 21 inches. Round your answer to the
nearest whole number.
C = (π)(d)
21 in.
C = (π)(21)
C = 65.97344573…
The circumference of the
C = 66 inches
circle is 66 inches.
Practice Problems
• Find the circumference of each circle.
Round your answers to the nearest tenth.
1.
5 ft.
C = 2π(r)
C = 2π(5)
C = 10 π
C = 31.41592654…
C = 31.4 feet
2.
70 in.
C = π(d)
C = π(70)
C = 219.9114858…
C = 219.9 inches
Word Problem
The Belknap shield volcano is located in
the Cascade Range in Oregon. The
volcano is circular and has a diameter of 5
miles. What is the circumference of this
volcano? Round your answer to the
nearest tenth.
C = π(d)
C = π(5)
C = 15.70796327…
C = miles