Circles: Finding Circumference

Circles: Finding Circumference

Important Circle Vocabulary

• Radius: a line segment from the center of a circle to the outside

• Diameter: a line segment going across an entire circle, passing through the middle

• Circumference: the distance around the outside of the circle, its perimeter

Radius:

This shows a circle with a radius of 50 cm, the distance from the center to the outside of the circle

Multiple Choice

What is the radius of this circle?

• 36 cm

• 9 cm

• 18 cm

• 12 cm

Diameter:

This image shows a circle with a diameter of 12 inches, the distance from one side to the other going through the center

Multiple Choice

What is the diameter of this circle? 

• 10 cm

• 5 cm

• 2.5 cm

• 50 cm

Radius and Diameter

• The diameter is twice the radius so: Diameter = Radius x 2

• The radius is half the diameter so: Radius = Diameter divided by 2

Multiple Choice

This circle shows the diameter, what is the radius of this circle?

• 12 cm

• 10 cm

• 5 cm

• 6 cm

Multiple Choice

This image shows the radius of a circle, what is the circles diameter?

• 10 m

• 5 m

• 12 m

• 15 m

Circumference of a Circle

This image shows that circumference is the distance around the entire circle - the red lines length

Circumference Formula #1

Since a circle has curved sides, we must use a mathematical formula to find the Circumference there are two formulas we can use

Circumference Formula #1

 $\pi\ =\ 3.14$ 

In this formula we multiply (3.14) times the diameter of the circle to find the circumference

Circumference Formula #2

Because diameter is radius times 2, we can also use this formula where we multiply pi (3.14) times the radius and also times 2.

Calculating Circumference

We use the formula to calculate the circumference: 

 $C\ =\ \pi d$  
 $C\ =\ 3.14\ x\ 12\$  
 $C\ =\ 37.68\ inches$  

Multiple Choice

 $C\ =\ 2\pi r$  

 $C\ =\ \pi d$      
What is the circumference of this circle? round to the nearest hundredth and use 3.14 for  $\pi$  

• 19.84 inches

• 9.42 inches

• 18.84 inches

• 17.88 inches

Multiple Choice

 $C\ =\ \pi d$ 

  $C\ =\ 2\pi r$  

What is the circumference of this circle?  use 3.14 for  $\pi$  and round to the nearest hundredth. 

• 56.52 in

• 55.25 in

• 66.56 in

• 28.26 in

Multiple Choice

What is the circumference of this circle?

• 50.25 in

• 25.24 in

• 60.34 in

• 56.52 in

Multiple Choice

What is the diamter of this circle?

• 3.5

• 7

• 14

• 6

Multiple Choice

What is the diameter of this circle?

• 36.8

• 75

• 73.6

• 74.5

Multiple Choice

What is the radius of this circle? 

• 18

• 17

• 36

• 19