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1.
A geometric figure.

In mathematics, an annulus (the Latin word for "little ring", with plural annuli) is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. It is also used to describe the area between two concentric circles. The adjectival form is annular (for example, an annular eclipse).

The area of an annulus is the difference in the areas of the larger circle of radius R and the smaller one of radius r:

A = \pi R^2 - \pi r^2 = \pi(R^2 - r^2)\,.
Whats an Annulus?
An annulus consists of multiple circles that represents a ring.
Por David Faustino 24 de noviembre de 2012