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

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