Annulus

The region in common to two concentric Circles of Radii $a$ and $b$. The Area of an annulus is

$$A_{\rm annulus} = \pi (b^2-a^2).$$

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

References

Pappas, T. ``The Amazing Trick.'' The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra, p. 69, 1989.