The region in common to two concentric Circles of Radii and . The Area
of an annulus is
In the above figure, the area of the circle whose diameter is tangent to the inner circle and has endpoints at the outer circle is equal to the area of the annulus.
See also Chord, Circle, Concentric Circles, Lune (Plane), Spherical Shell
Pappas, T. ``The Amazing Trick.'' The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra, p. 69, 1989.