Circumference divided by diameter = pi. Radius is diameter divided
http://www.chacha.com/question/whatistheradius...

1. Write down the circumference of the circle for which you wish to calculate the radius. 2. Divide the circumference by pi, which is approximately 3.14. As an example, divide a circumference
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C = 2pi(r) 10 = 2pi(r) 10/(2pi) = r. 5/pi = r. The radius is 5/pi, which is about 1.59.
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