The formula for the area of a circle, A, in terms of its radius, r, is:

  A = πr²

We can rearrange this formula to make r the subject.

The formula for the radius of a circle, r, in terms of its area, A, is:

Example

Calculate the radius, r cm, of a circle which has an area of 100 cm². 

Give your answer to 3 sig. figs.

Answer

r² = 100 ÷ π = 31.831

r = √31.831 = 5.641896 ≈ 5.64 cm (3 s.f.)

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