# annulus

An annulus is the region between the smaller and the larger of two circles that share a common center. 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*= π

*R*

^{2}- π

*r*

^{ 2}= π

*R*

^{2}- π

*r*

^{2}.