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Maths

Circles

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Circumference

We have already seen that circumference ÷ diameter = π

Therefore, circumference = π × diameter. This can be written as:

C = π d

The diameter is twice the length of the radius. This formula can be written as:

C = 2 π r

Question

A circle has a diameter of 10cm. Find the length of its circumference. (Use π = 3.14)

A circle with a 10cm diameter and 5cm radius
Answer

Using C =πd
C = 3.14 × 10
C = 31.4 cm

Using C = 2 πr
C = 2 × 3.14 × 5
C = 31.4cm

Question

Anish and Becky each have a circular pond in their garden.
Anish's pond has a diameter of 6m.
Becky's pond has a diameter of 3m.
Anish says that the circumference of his pond is twice the circumference of Becky's pond.

a) Find the circumference of each pond (use π = 3.14).
b) Is Anish's statement correct?

Answer

a) The circumference of Anish's pond is 18.84m and the circumference of Becky's pond is 9.42m.
b) It is clear from the answers to a) that Anish's statement is correct.

Activity

Circles activity

Going round in circles? Try this activity.

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