Question

What is Annulus (Plural : annuli)?

Answer

Annulus is the region enclosed by two concentric circles.

$\mathrm{Radius\; of\; small\; circle}=r$
$\mathrm{Radius\; of\; bigger\; circle}=r+R$
$\mathrm{Area\; of\; Annulus}=\pi {\left(\mathrm{r+R}\right)}^2–{\mathrm{\pi r}}^2$

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