10'3 Circumference of a Circle

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10.3 Circumference of a Circle
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Finding the Circumference of a Circle
Vocabulary
A circle is the set of all points in a plane that
are the same distance from a point called the
center.
The radius, r, is the distance from the center to
any point on the circle.
The distance across the circle through its center
is the diameter.
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Using the Radius to find the Circumference
If you are asked to find the circumference of a
circle and only the radius is given, follow these
steps.

• Formula
• Substitute
• Simplify

6 mm
3 yd
C 2?r
C 2?r
C 2(3.14)6
C 2(3.14)3
C 12(3.14)
C 6(3.14)
C 37.68 mm
C 18.84 yd
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Using the Diameter to Find the Circumference
You may also be asked to find the circumference
of a circle and only the diameter is given.
Follow these steps to solve the problem.

• Formula
• Substitute
• Simplify

4 ft
7 cm
C ? d
C ? d
C 3.14(4)
C 3.14(7)
C 12.56 ft
C 21.98 cm
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Finding the Circumference
When asking for the circumference, you may use
either formula.
9 mi
8m
25 ft
6 in
C ? d
C 2?r
C ? d
C 2?r
C 3.14(18)
C 2(3.14)12.5
C 3.14 (8)
C 2(3.14)6
C 25(3.14)
C 25.12 m
C 56.52 mi
C 12 (3.14)
C 78.5 ft
C 37.68 in
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Now You Try!
9 yd
2 m
16 in
C ? d
C 2 ? r
C ? d
C 3.14 (16)
C 2 (3.14) 9
C 3.14 (4)
C 50.24 in
C 18 (3.14)
C 12.45 m
C 56.52 yd