Area and Circumference of a Circle

C = 2 π r

A = π r^{2}

Both formulas use the radius. If you're given the diameter, cut it in half first.

Worked examples

$C = 2 π (3) = 6 π \approx 18.85$

$A = π (3^{2}) = 9 π \approx 28.27$

The radius is 5.

$C = 10 π \approx 31.42$

$A = 25 π \approx 78.54$

$π r^{2} = 49 π$ , so $r = 7$ .

$C = 2 π (7) = 14 π$

Diameter 10, so the area is $π (1 0^{2}) = 100 π$ .

The formula needs the radius. With $r = 5$ , the area is $25 π$ , which is a quarter of that wrong answer.

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