Circle basics 1

Circles

Calculating Areas and Circumferences

Formula you need to know

• Area -  $\pi r^2$  

• Circumference -  $2\pi r$  

• Diameter -  $2r$   

• (Make a note of these if you need) 

Area

 $\pi r^2\ or\ 3.14\ \times radius^2$  

So to calculate the area we simply multiply the radius by itself, then multiply by 3.14... for example, we have a circle with a radius of 3 cm... 

We take  $\pi$   and multiply it by  $3^2$  
3.14 X  $3^2$   $=$  28.274 $cm^2$       (dont forget the  $cm^2$  as this is the area and all areas are $unit^2$  

The radius of a circle

The measurement from the middle of the circle to the outside of the circle

Fill in the Blanks

The radius is half of the

Multiple Choice

(Calculater if need) You have a radius of 7mm, what is the area?

• 153.9 cm

• 153.9 $mm^2$

• 7mm

• 10 $mm2$

Calculate the circumference

 $2\pi r$  or  $2\ \times\ 3.14\ \times\ radius$  
If we know the radius or the diameter, we can calculate the circumference with this formula for example, if we have a radius of 4cm, all we have to do is multiply 2 X 3.14 X 4cm which would give us a circumference of 25.12cm

The circumference

• The full measurement of the outside of the circle

Multiple Choice

SO FAR WE KNOW..... The radius is....?

• The perimeter of the of the circle

• The area of the circle

• The measurement right across the circle

• Measurement from the middle of the circle to the outside

Multiple Choice

The Circumference is ... ?

• The middle of the circle

• The perimeter of the circle

• The middle to the edge of the circle

• The amount of corners

So what about the diameter?

The diameter is the full length across the circle... simply the radius multiplied by 2

Multiple Choice

So if the radius is 16cm, what is the diameter?

• 32cm

• 32mm

• 45cm

• 1cm

Multiple Choice

If the diameter of a circle was 54mm, what would the radius be?

• 21mm

• 27M

• 27mm

• 13mm

How do we find the Circumference?

:)

 $c\ =\ \pi d$  

Easy?

Poll

What is the Circumference if the diameter is 12cm?

• 37.69cm

• 42.31cm

What if we were only given the radius?

Poll

A radius of 8 is given, what is the Circumference?

• 17.2cm

• 50.3 cm

QUIZ

Multiple Choice

A circle has a radius of 18cm, what is the area?

• 791cm

• 89m

• 1017cm

• 1017

  $cm^2$

Multiple Choice

And what would be the circumference if we had a radius of 18cm?

•  $113.04cm2$  
  

• 110cm

• 113.04cm

• 76.03cm

Multiple Select

What would be the area of a circle with a radius of 4m?

• 50.42m

• 50.24cm

• 50.24m

• 0

Multiple Choice

What about the circumference if we had a diameter of 4m?

• 12.56m

• 13.4

• 10.3

• 7989044mm