CircumferenceFinding the

and Area of

Circles

Geometry:

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

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

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

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

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 ?

3.14 × 18What if you are only given the

radius?

C ≈ 56.5 in

Circumference of Circles

Diameter = 18 in

Remember!

𝑪=𝝅×𝒅

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

C = 2 × 3.14 × 12C ≈ 75.4 in

Circumference of CirclesRadius = 12 in

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

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

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

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

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

Extension!Here’s how to find the diameter of circle

if you know its circumference.

𝒅=¿

?→

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