1. The Circle Introduction to circles Let’s investigate… Circumference
Circumference examples

2. Starter Questions
7cm

3. Main part of a Circle Main parts of the circle radius Diameter
Circumference

4. How can we measure the circumference?
Let’s investigate…
circumference
We can use a ruler to measure the diameter.
How can we measure the circumference?

5. circumference ÷ diameter
Let’s investigate…
Look at the column
circumference ÷ diameter 3
circumference ÷ diameter is roughly
There isn’t an exact answer for this.
It actually goes on forever!
In 1989 a computer worked it out to 480 million decimal places. …
We’ll stop here since it would stretch for 600 miles if we printed them all!

6. Let’s Investigate...
Circular Item
Circumference
Diameter Π

7. If it goes on for ever how can I write it down?
The Circumference
If it goes on for ever how can I write it down?
Mathematical
Genius!
We use the Greek letter
instead.
This is called pi.

8. The Circumference Circumference = x diameter C = d
So circumference ÷ diameter =
By re-arranging this we get:
Circumference =
x diameter
C = d

9. The Circumference
When doing circle calculations, you will normally use a calculator.
Some calculators have a button like this:
This button stores to 8 or 9 decimal places which is more than accurate enough!
If your calculator doesn’t have
Then use 3.14 instead.

10. Example 1 6cm C = d C = x 6 C = 18.8cm (1 d.p.)
Press
Then x 6 =
What is the circumference of this circle?

11. Example 2 C = d d = 2 x 5 = 10cm 10cm C = x 10 5cm C = 31.4cm (1 d.p.)
What is the circumference of this circle?
Remember:
diameter = 2 x radius

12. Go back to the Circles worksheet and use
The Circumference
Go back to the Circles worksheet and use
to work out the circumference of each circle.
C = d

13. There is a much more accurate way!
Area of a circle 1 ? 2 3 4 5 6 7
Mathematical
Genius! 8
To find the area we could try counting the squares inside the circle…
There is a much more accurate way!

14. There is a special formula for the area of a circle.
x radius
A = r²
Remember:
r² means r x r

15. Example 1 A = r² A = x 4 x 4 4m A = 50.3m² (1 d.p.) Press
Then x 4 x4 =
What is the area of this circle?

16. Example 2 ? A = r² r = ½ x 14 = 7cm 7cm A = x 7 x 7 14cm
Don’t
forget!
r =
½ x 14
= 7cm ?
7cm
A = x 7 x 7
14cm
A = 153.9cm² (1 d.p.)
Press
Then x 7 x 7 =
What is the area of this circle?

17. Example 3 ? A = r² 24m r = ½ x 24 = 12m 12m A = x 12 x 12
Don’t
forget!
24m
r =
½ x 24
= 12m
12m ?
A = x 12 x 12
A = 452.4m² (1 d.p.)
What is the area of
this semi-circle?
Area of semi-circle
= ½ x 452.4
=226.2m²
First work out area of full circle.
A semicircle is half a circle.