Annulus

A plane figure that consists of the area between a pair of concentric circles is called an annulus.

Introduction

When two circles are arranged such a way that they both have a common center (or centre), the two circles are called the concentric circles. The intersection between them forms a region and it is called an annulus.

An annulus is a circular ring shape object geometrically. Hence, it is also called a circular ring.

Area

Let $R$ and $r$ represent the radius of a big and small circles, and the area of the annulus is denoted by $A$. The area of a circular ring is written algebraically as follows.

$A \,=\, \pi(R^2-r^2)$

Proof

Learn how to derive the area of an annulus geometrically in algebraic form by two concentric circles.