Lesson 8-5 Areas of Circles

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Lesson 8-5Areas of Circles
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You will learn

• To find the areas of circles,

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The Area of a circle is the number of square
units contained inside the circle.
Example Estimate the number of square units
inside the circle.
Area of a circle
There are about 12 squares plus the 4 parts that
are approximately ¼ of a square each.
There are about 13 square units inside this
circle.
It is hard to always count the squares so it
would be a good idea to use a formula.
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Most of the shapes you have you have investigated
in this chapter could be divided into rectangles
or triangles. Can a circle be divided into
rectangles or triangles?
Area of a circle
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• Make a large circle, cut it and fold the circular
  region in half.
• Fold it in half a second time, then a third time
  and a fourth time.
• Unfold your circle and cut it along the folds
  into 16 wedges.

Area of a circle
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• Arrange the wedges in a row, alternating the tips
  up and down to form a shape that resembles a
  parallelogram.

Area of a circle
r
 
r
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Attached the two figures then seem as a
parallelogram
Height radius
Area of a circle
Length of the base
It looks very much like a parallelogram!
Therefore its area base x height
Length of the base ½ of the circumference
 
 
 
 
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Area of a circle
Area of a circle
r
r
r
 
A ?r2
Area of Circle Units2
Radius of circle
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How to find the area of a circle given the radius
Find the area of the circle below
If you have the radius square it and then
multiply by 3.14.
Area of a circle
State the formula
Substitute the values for p and r.
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How to find the area of a circle given the
diameter
Find the area of the circle below
If you have the diameter Divide the diameter by
2 to get the radius.
Area of a circle
Square the radius and then multiply by 3.14 (?).
State the formula
Substitute the values for p and r.
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ExampleHow much more pizza is in a 12in
diameter pizza than in a 10in diameter pizza?
Area of a circle
A10 ?r2 52? 25? 78.5in2
A12 ?r2 62? 36? 113.1in2
36? - 25? 11? 34.6in2