Look at my head! It’s almost a perfect circle! Look at my head! It’s almost a perfect circle!
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Transcript of Look at my head! It’s almost a perfect circle! Look at my head! It’s almost a perfect circle!
Geometry:
Finding the Circumference
and Area of
Circles
Look atmy head! It’s almost
a perfect circle!
In this lesson, you will learn the formulas for:
Circumference – the distancearound a circle.
Area – the number of square
units a circle contains.
Area
A few more terms you need to know…
Radius – a line that goesto the center of a circle fromany edge. Diameter – a
line thatgoes all the way across acircle through its center.
Pi or π – a number (3.14) that we use tofigure out the circumference and area of circles.
So how do wefind the circumference
of a Grubby Dub’s Pizza?
First, we need to know the diameter of the pizza.
Diameter = 18 in
And then we use the following formula:
In the big leagues, we write that formula like this:
Diameter = 18 inches
C = 3.14 × 18That doesn’tseem bad, but whatif you’re only given
the radius?
That’s a great question! Check this out!
C ≈ 56.5 in
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!
Isn’t doubling like multiplying by 2?
That’s right! Someone’s on top of their game today.
Radius = 12 in
It’s a gift.
Radius = 12 in
or
We’ll use this formula here!C = 2 × 3.14 × 12 Now, let’s
take a look at finding the area of a
circle!
C ≈ 75.4 in
Let’s find the area of this
stained glass window!
Radius = 2 ft
or
Remember! You MUST always follow
order of operations and simplify exponents
FIRST!4
𝐴≈12.6 ft2
Do youthink you could
find the area of say…my face?
That’s a little awkward,
but I suppose we could!
Sweet!How do we
do this?
First, we need to measure the width of your
face.
It looks like the diameter of your face is about 13
inches.
𝐴=𝜋 𝑟2
Is that good?
Remember! To find the area, of a circle we need the radius.
Well…umm…
13 ÷ 2 = 6.5 inSo, divide the diameter by 2!
𝐴 = 3.14× (6.5) 2
𝐴≈132.7 ¿2That’s quite a
dome you have there!
r = 6.5 in
𝐴 = 3.14×42.25