Radius
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The radius of a circle is the distance from the center to any point on the circle. Identical definitions hold for the sphere and hypersphere. The plural form of radius is radii.
The radius of a circle is often denoted using R or r.
![[asy]size(150); pair O=origin, P=dir(30); D(unitcircle); D(O--P); MP("O",D(O),NNW); MP("P",D(P),NE); MP("r",midpoint(O--P),NNW);[/asy]](http://latex.artofproblemsolving.com/d/1/2/d12fe09918e0b15cda7055e7772d2844cedbc269.png)
