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See also: Common Measurement Units - Length, Area & Volume 

Annulus (Circle Ring)

An annulus is the region between two concentric circles. The area of the annulus formed by two circles of radii ri and ro is given by

The mean radius rm is defined by the following equation:


It can be shown that the area of the annulus is equal to the area of the circle whose diameter is tangent to the inner circle (point C) and whose radius is the line segment between point C and the intersection of the tangent and the outer circle (points A and B).


Last Update: 2005-Feb-03