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.
- Least Prime Factor
- Least Squares Fitting
- Least Squares Fitting--Exponential
- Least Squares Fitting--Logarithmic
- Least Squares Fitting--Power Law
- Least Upper Bound
- Lebesgue Constants (Fourier Series)
- Lebesgue Constants (Lagrange Interpolation)
- Lebesgue Covering Dimension
- Lebesgue Dimension
- Lebesgue Integrable
- Lebesgue Integral
- Lebesgue Measurability Problem
- Lebesgue Measure
- Lebesgue Minimal Problem
- Lebesgue-Radon Integral
- Lebesgue Singular Integrals
- Lebesgue-Stieltjes Integral
- Lebesgue Sum
- Le Cam's Identity
- Leech Lattice
- Lefshetz Fixed Point Formula
- Lefshetz's Theorem
- Lefshetz Trace Formula
- Leg