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.)
Let's have a go at some questions.
View the activity introduction for more information on the topic
Read the question aloud
Open the accessibility toolbar to change fonts and contrast, choose a different language, use a ruler and more
Question
Do you want to skip questions and finish?
Hold up, it looks like there may be an issue with your spelling. Why not try it again?
You need to check your answer before you proceed. Are you sure you want to skip?
Teacher explanation