How to calculate the area of a circle'

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How to calculate the area of a circle.
Its as easy as pi.
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Lets first make sure that we understand the
difference between circumference and area.
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The circumference of a circle is the perimeter of
the circle.
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Imagine that the circle is straightening itself
out.
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The length of this line segment is the
circumference of the circle.
314 cm
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The circumference is the same length as 3
diameters plus .14 of another diameter.
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So, circumference diameter x 3.14
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Does this look familiar?
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O.K., now its time to move forward with some new
stuff.
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How in the world would you find the area of a
circle?
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Remember, area is always measured in square units.
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Remember that the area of a rectangle is length x
width because youre calculating the total number
of squares inside of the rectangle.
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4
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Thats fine and dandy, but a circle is not a
polygon. It does not have straight sides it has
curves.
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How are we going to get around these curves?
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Imagine chopping up the circle as if it were a
pizza.
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Now, lets rearrange our pizza into another
shape.
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PRESTO!
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Great Mr. Dunlap! But what in the world is this?
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Believe it or not, this is really our friend
the parallelogram. And, we know how to calculate
the area of a parallelogram.
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Rats! He always has an answer for everything.
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Area Base x Height
Height
Base
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To find the area of the circle (which is now a
parallelogram), we just need to multiply the Base
by the Height.
Height
Base
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Wait a minute! The height of this parallelogram
is really the radius of the circle.
Radius
Base
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Wait a minute! The Base is really 1/2 of the
circumference.
Radius
1/2 of Circumference
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Wait a minute! The circumference is really
Diameter x ?
Radius
1/2 of Diameter x ?
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Wait a minute! 1/2 of a Diameter is really a
Radius.
Radius
Radius x ?
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So if we multiply the Base x Height
Height
Base
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We are really multiplyingRadius x Radius x ?
Radius
Radius x ?
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Practice Time!
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1) Now lets try this formula. Find the area of
this circle.
5 cm
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5 x 5 x 3.14 78.5 square cm
5 cm
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2) Find the area of this circle.
6 cm
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6 x 6 x 3.14 113.04 square cm
6 cm
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3) Find the area of this circle.
9 cm
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9 x 9 x 3.14 254.34 square cm
9 cm
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4) Find the area of this circle.
20 cm
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10 x 10 x 3.14 314 cm2
Make sure that you use the radius of the circle.
20 cm
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5) Find the area of this circle.
14 cm
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7 x 7 x 3.14 153.86 cm2
Make sure that you use the radius of the circle.
14 cm
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6) Find the area of this circle.
22 cm
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11 x 11 x 3.14 379.94 cm2
22 cm
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Area Radius x Radius x ?
Its as easy as pi.