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This activity is about finding the circumferences of circles.
The circumference of a circle is its perimeter or the distance around its edge.
You need to learn these two important formulae for a circle of radius, r or diameter, d:
Circumference of a circle = 2πr
Circumference of a circle = πd
This is because the diameter of a circle is two times the length of the radius: 2 x r = d
The value of π (pi) is 3.142.... but is an infinite number, so the best way to do any calculations with pi is to use a scientific calculator that has a special π button.
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Example 1
Find the circumference of a circle of radius 6 cm.
Answer
Circumference = 2 × π × r = 2 × π × 6 = 12 × π = 37.7 cm (1 decimal place)
Example 2
Find the circumference of a circle of diameter 6 cm.
Answer
Circumference = π × d = π × 6 = 18.8 cm (1 decimal place)
Use a scientific calculator to work out your answer because you can have a more accurate number for pi (π ).
If you don't have one in the house, you can access one online.
Let's have a go at some questions.
