 Place a piece of string round the circle and then open it out flat:

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Place a piece of string round the circle and then open it out flat:
The circumference is the distance around a circle. Place a piece of string round the circle and then open it out flat: diameter Measure the length of the line: Measure the diameter of the circle. What is the connection between the diameter and the circumference?

Measure the distance around the outside of each circle
Measure the distance around the outside of each circle. Measure the diameter of each circle and complete the table. Circle Circumference Diameter A B C D

Circle D Circle B Circle A Circle C

The circumference is the distance around a circle.
diameter 1 2 3 0.1 circumference

Circumference is equal to roughly 3 x diameter……
Circumference is equal to roughly 3 x diameter…….and this works or every circle!

It’s not easy to use a ruler to measure the distance around a circle.
A formula for the circumference makes finding it easy. A formula for the connection between the circumference and the diameter is The symbol, , (pi) is said as pie, and is 3.14 (to 2 dp). This is how accurate we need to be on the GCSE calculator paper.

How to find circumference given the diameter of a circle
Find the circumference of the circle below: If you have the diameter: multiply the diameter by 3.14 (). State the formula Substitute the values for p and d.

Questions: Given diameter, find the circumference

How to find circumference given the radius
Find the circumference of the circle below: If you have the radius: multiply this by 2 first to get the diameter. Then multiply the diameter by 3.14 (). State the formula Substitute the values for p and d.

Review

What’s the formula? Calculate the missing numbers in the table below using the circumference formula below. Use 3.14 as an approximation for p. 13.502 6.8 5 C (cm) d (cm) r (cm) 10 31.4 3.4 21.352 2.15 4.3

Plenary You should know the words used in a circle to include the diameter, radius, circumference and p. Know that p is roughly 3.14. Know and use the formula for the circumference of a circle. or

Find the circumference of each circle below.

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