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Maths

Circles

Area of a circle

There is only one formula for the area of a circle:

A = π r2

We must therefore remember to use the radius each time.

Question

Find the area of a circle with a diameter of 20 cm. Use the π button on your calculator, and give the answer correct to 1 dp.

image: circle, diameter 20cm

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Answer

We have been told that the diameter is 20 cm. Therefore, the radius is 10 cm.

A = π r2

A = π × 10 × 10 = 314.159... cm2

A = 314.2 cm2 (1 dp)

Remember

  • Units of area are always written as squares. For example, cm2, m2 etc.
  • πr2 means π × r × r (only the r is squared).

Now try this last question.

Question

Find the area of a circle with a radius of 12 cm.

Use the π button on your calculator, and give your answer correct to 3 significant figures (s.f.).

image: cirle radius 12cm

toggle answer

Answer

The answer is 452 cm2.

You need to calculate π × 12 × 12 = 452.3893... and then round your answer to 3 s.f.

Back to Geometry and measures index

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