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annulus
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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}
.
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