- Irina Boyadzhiev
Theorem: All oval rings swept out by a line segment of given length that is tangent to every point of a smooth closed curve have equal areas, regardless of the size or shape of the inner curve. Moreover, the area depends only on the length of the tangent segment and is equal to , the area of a circular disk of radius , as if the tangent segment was rotated about a single point. Reference: Mamikon Mnatsakanian, "Annular Rings of Equal Area," Math Horizons, 5(3), 1997 pp. 5–8.