Title: How to calculate the area of a circle'
1How to calculate the area of a circle.
Its as easy as pi.
2Lets first make sure that we understand the
difference between circumference and area.
3The circumference of a circle is the perimeter of
the circle.
4Imagine that the circle is straightening itself
out.
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8The length of this line segment is the
circumference of the circle.
314 cm
9The circumference is the same length as 3
diameters plus .14 of another diameter.
10So, circumference diameter x 3.14
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15Does this look familiar?
16O.K., now its time to move forward with some new
stuff.
17How in the world would you find the area of a
circle?
18Remember, area is always measured in square units.
19Remember that the area of a rectangle is length x
width because youre calculating the total number
of squares inside of the rectangle.
2
4
20Thats fine and dandy, but a circle is not a
polygon. It does not have straight sides it has
curves.
21How are we going to get around these curves?
22Imagine chopping up the circle as if it were a
pizza.
23Now, lets rearrange our pizza into another
shape.
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31PRESTO!
32Great Mr. Dunlap! But what in the world is this?
33Believe it or not, this is really our friend
the parallelogram. And, we know how to calculate
the area of a parallelogram.
34Rats! He always has an answer for everything.
35Area Base x Height
Height
Base
36To find the area of the circle (which is now a
parallelogram), we just need to multiply the Base
by the Height.
Height
Base
37Wait a minute! The height of this parallelogram
is really the radius of the circle.
Radius
Base
38Wait a minute! The Base is really 1/2 of the
circumference.
Radius
1/2 of Circumference
39Wait a minute! The circumference is really
Diameter x ?
Radius
1/2 of Diameter x ?
40Wait a minute! 1/2 of a Diameter is really a
Radius.
Radius
Radius x ?
41So if we multiply the Base x Height
Height
Base
42We are really multiplyingRadius x Radius x ?
Radius
Radius x ?
43Practice Time!
441) Now lets try this formula. Find the area of
this circle.
5 cm
455 x 5 x 3.14 78.5 square cm
5 cm
462) Find the area of this circle.
6 cm
476 x 6 x 3.14 113.04 square cm
6 cm
483) Find the area of this circle.
9 cm
499 x 9 x 3.14 254.34 square cm
9 cm
504) Find the area of this circle.
20 cm
5110 x 10 x 3.14 314 cm2
Make sure that you use the radius of the circle.
20 cm
525) Find the area of this circle.
14 cm
537 x 7 x 3.14 153.86 cm2
Make sure that you use the radius of the circle.
14 cm
546) Find the area of this circle.
22 cm
5511 x 11 x 3.14 379.94 cm2
22 cm
56Area Radius x Radius x ?
Its as easy as pi.