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

Let’s practice!Find the

circumference for number’s 1 and 2.

11 ft1.

2.

7 m

3.

4.

Find the area for number’s 3 and 4.

C ≈ 69.1 ft

C ≈ 22 m

Round all answers to the nearest

tenth. 7 in A ≈ 38.5 in2

12 kmA ≈ 452.2 km2