Annulus
The region in common to two concentric Circles of Radii 
and 
. The Areaof 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 outercircle 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.
- Absorption Law
- Abstraction Operator
- Abundance
- Abundant Number
- Acceleration
- Accidental Cancellation
- Accumulation Point
- Achilles and the Tortoise Paradox
- Ackermann Function
- Ackermann Number
- Acnode
- Acoptic Polyhedron
- Action
- Acute Angle
- Acute Triangle
- Adams-Bashforth-Moulton Method
- Adams' Method
- Addend
- Addition
- Addition Chain
- Addition-Multiplication Magic Square
- Additive Persistence
- Adem Relations
- Adequate Knot
- Adiabatic Invariant