Circumference and Area of Circles

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Circumference and Area of Circles
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Parts of a Circle

• Radius-Straight line from the middle to the
  outside of a circle

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• Diameter-a line that cuts the circle in half from
  end to end

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Circumference

• Circumference is the distance around the circle.
  Like the perimeter.

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Pi

• Pie is a special number. It is the ratio of the
  circumference and the diameter

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• Pi goes on forever!!!!!!!!!!!
• We approximate it using 3.14

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Finding Circumference
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Finding Circumference
6 3.14 18.84 in.
6?? in. would be the exact answer, but we are
going to estimate using ?? 3.14
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What if you have the radius?
The radius is half of the diameter
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How Would You Find the Diameter if You Only Had
the Circumference?

• Its easy!
• When given circumference, to find diameter just
  divide by pi.

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Circumference ??d
21.98 3.14d
C 21.98 cm
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Circumference ??d
C 37.68 m
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Circumference ??d
C 47.10 m
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I thought penguins were smart?
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Area of a Circle
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Area of a Circle

• The Formula

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Example
3.14 36 113.04 in²
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Try
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Try
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What if you are given the diameter?

• How can you find the radius if you only have the
  diameter?
• The radius is half of the diameter
• Diameter 8 then radius 4

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Example
Radius 3
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Try
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Finding Radius and Diameter from Area

• Just like you could find radius and diameter from
  circumference, you can find the radius and
  diameter from area by filling in known values and
  solving.

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Example
Area ??r²
 
 
Area 78.50 cm² Radius Diameter
 
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Area 314.00 cm² Radius Diameter
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Area 200.96 cm² Radius Diameter
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Closure

• What is the formula for finding the area of a
  circle?

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Closure

• How can you find the radius if you only have the
  diameter?

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Closure

• What is the diameter?

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Closure

• What is the radius?

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Closure

• What is circumference?

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Closure

• What is the relationship between the radius and
  the diameter?