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A radius of a circle (or sphere) is a line segment with one endpoint at the center, and the other endpoint at the boundary. The word is also used to mean the length of such a segment.

All the radii of a particular circle (or sphere) are equal in length. The length of the radius of a circle is equal to

C 2 π ,

where C is the circumference of the circle, and π is pi , an irrational number approximately equal to 3.14 .

The radius is also used in the calculation of the area of a circle. The formula is

A = π r 2 .

See also diameter .