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Circumference
of a
Circle
Part II |
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Unit 2
> Lesson 2 of 6 |
Circumference, diameter and radii are measured in linear units, such as inches and centimeters.
A circle has many different radii and many different diameters, each passing through the center. A
real-life example of a radius is the spoke of a bicycle wheel. A 9-inch pizza is an example
of a diameter: when one makes the first cut to slice a round pizza pie in half, this cut is the
diameter of the pizza. So a 9-inch pizza has a 9-inch diameter.
Let's look at some examples of finding the circumference of a circle. In these
examples, we will use  = 3.14 to
simplify our calculations.
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 ![[IMAGE]](images/radii.gif)
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| Example 1: |
The radius of a circle is 2 inches. What is the diameter?
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![[IMAGE]](images/example1.gif)
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| Solution: |
![[IMAGE]](images/diam_formula.gif) |
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 = 2 · (2 in) |
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= 4 in |
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| Example 2: |
The diameter of a circle is 3 centimeters. What is the circumference?
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![[IMAGE]](images/example2.gif)
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| Solution: |
 |
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 = 3.14 · (3 cm) |
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= 9.42 cm |
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| Example 3: |
The radius of a circle is 2 inches. What is the circumference?
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![[IMAGE]](images/example3.gif) |
| Solution: |
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= 2 · (2 in) |
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= 4 in |
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= 3.14 · (4 in) |
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= 12.56 in |
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| Example 4: |
The circumference of a circle is 15.7 centimeters. What is the diameter?
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![[IMAGE]](images/example4.gif)
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| Solution: |
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15.7 cm = 3.14 · |
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15.7 cm ÷ 3.14 = |
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= 15.7 cm ÷ 3.14 |
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= 5 cm |
| Summary: |
The number is the ratio of the
circumference of a circle to the diameter. The value of is
approximately 3.14159265358979323846...The diameter of a circle is twice the radius.
Given the diameter or radius of a circle, we can find the circumference. We can
also find the diameter (and radius) of a circle given the circumference. The formulas
for diameter and circumference of a circle are listed below. We round
to 3.14 in order to simplify our calculations. |
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