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.

**References**

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

© 1996-9

1999-05-25