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CircumferenceFinding the
and Area of
Circles
Geometry:
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Circumference and area of CirclesIn this lesson, you will learn
how to find:
Area – the number of square units a circle contains
Circumference – the distance around a circle
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Radius – a straight line that goes from the center
of a circle to any edge.
Diameter – a straight line that goes all the way across a
circle through its center.
Some more terms you need to know:Circumference and area
of Circles
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Let’s Talk about Pi!
3.141592653589793238…
𝜋𝜋 𝜋𝜋Whenever you divide the distance around
a circle by its own diameter, you alwaysget a constant number called “Pi.”
Pi’s is non terminating and non repeating
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Examples of Pi
diameter = 2.4 cm
7.54 ÷ 2.4
Diameter= 7,926 mi
24,902 ÷ 7,926
Circumference of a quarter = 7.54 cm
Circumferenceof Earth = 24,902 mi
3.14
3.14
𝜋𝜋 𝜋𝜋
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First, we need to know the diameter of the pizza.
Diameter = 18 in
Then, we use the following formula:
Circumference of Circles
𝐶=𝜋 𝑑
What is the circumference of the pizza ?
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3.14 × 18What if you are only given the
radius?
C ≈ 56.5 in
Circumference of Circles
Diameter = 18 in
Remember!
𝑪=𝝅×𝒅
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Here, we’re given a radius, but we need a diameter to
find the circumference of the dart
board.
So, what should wedo to this radius?
If you said we need to “double” it, you’re right!
Circumference of CirclesRadius = 12 in
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C = 2 × 3.14 × 12C ≈ 75.4 in
Circumference of CirclesRadius = 12 in
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Find the circumference for each of the following circles. Round answers to the nearest tenth.
8 ft1.
3.
7 m
2.
4.
C ≈ 50.2 ftC ≈ 22 m
4 in C ≈ 12.6 in10.5 cmC ≈ 65.9 cm
Show Your Stuff!Use: 3.14 for
or
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Now, let’s find the area of this stained
glass window. Radius = 3 ft
or
Remember! Always follow order of operations and
simplify exponents FIRST!
𝐴=3.14×9
𝐴≈28.3 ft2
Area of Circles
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Do youthink we could
find the area of say…my face? How do we
do this?
First, we need to measure the width of your face.
Area of Circles
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It looks like the diameter of your face is
about 13 inches.
𝑨=𝝅 𝒓𝟐
Remember! To find the area, of a circle we need the
radius.
13÷2=6.5 𝑖𝑛So, divide the diameter by 2!𝐴 = 3.14× (6.5) 2
𝐴≈132.7 ¿2𝑟=6.5 𝑖𝑛𝐴 = 3.14×42.25
Area of CirclesDiameter = 13 in
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6 ft1.
3. 5.4 cm
2.
4.
9 in
14 km
Show Your Stuff!Find the area of each circle below. Round answers to the nearest tenth.
Use: 3.14 for
𝑨≈𝟏𝟏𝟑 𝒇𝒕𝟐
𝑨≈𝟔𝟏𝟓 .𝟒𝒌𝒎𝟐
𝑨≈𝟐𝟐 .𝟗𝒄𝒎𝟐
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Extension!Here’s how to find the diameter of circle
if you know its circumference.
𝒅=¿
?→
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𝑨=?
Here’s how to find the area of a circle if you know its circumference.
Extension!
m First, find the diameter:
m = 6 m 𝑨=𝝅 𝒓𝟐
𝒅=¿
𝐴=3.14×36 ≈113𝑚2