The Circle

1
The Circle

• Special words
• Finding the circumference
• Finding the area

2
Special Words
3
Objective

• Be able to find the circumference of a circle.
• Be able to find the perimeter of shapes with
  circular parts.

4
Finding the circumference
The formula for the circumference is
Sometimes we are given the diameter
C pd
8cm
C 3.14 x 8 25.12 cm
p 3.14 (2dp)
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Finding the circumference
C pd
C 3.14 x 14 43.96 cm
14cm
p 3.14 (2dp)
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Finding the circumference
Sometimes we are given the radius, r
Now the formula for the circumference is
C 2pr
But this is just half of the diameter, so we use
2r instead of d
5cm
C 2 x 3.14 x 5 31.4 cm
p 3.14 (2dp)
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Finding the circumference
C 2pr
C 2 x 3.14 x 3 20.09 m
3m
p 3.14 (2dp)
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What is the perimeter?
3m
12m
The perimeter is the distance around the outside
of the shape
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What is the shape made of?
10
What is the shape made of?
11
So really we have two shapes A rectangle and a
circle
So this part gives us 12m 12m 24m
12m
C 2pr
For this part, we find the circumference
3m
C 2 x 3.14 x 3
So the total perimeter is 24m 18.84m 42.84m
18.84m
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Objective

• Be able to find the area of a circle.
• Be able to find the area of compound shapes with
  circular parts

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Finding the area
Usually we are given the radius, r
The formula for the area of a circle is
A pr2
A 3.14 x 72 3.14 x 7 x 7 153.86 cm2
7cm
p 3.14 (2dp)
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Finding the area
If you are given the diameter, simply half this
to find the radius first.
The formula for the area of a circle is
A pr2
A 3.14 x 42 3.14 x 4 x 4 50.24 m2
Here r 4m
8m
p 3.14 (2dp)
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