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Ellipse to Hyperbola: “With This String I Thee Wed”

Apostol, Tom M. and Mnatsakanian, Mamikon A. (2011) Ellipse to Hyperbola: “With This String I Thee Wed”. Mathematics Magazine, 84 (2). pp. 83-97. ISSN 0025-570X.

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We introduce a string mechanism that traces both elliptic and hyperbolic arcs having the same foci. This suggests replacing each focus by a focal circle centered at that focus, a simple step that leads to new characteristic properties of central conics that also extend to the parabola. The classical description of an ellipse and hyperbola as the locus of a point whose sum or absolute difference of focal distances is constant, is generalized to a common bifocal property, in which the sum or absolute difference of the distances to the focal circles is constant. Surprisingly, each of the sum or difference can be constant on both the ellipse and hyperbola. When the radius of one focal circle is infinite, the bifocal property becomes a new property of the parabola. We also introduce special focal circles, called circular directrices, which provide equidistance properties for central conics analogous to the classical focus-directrix property of the parabola. Those familiar with paperfolding activities for constructing an ellipse or hyperbola using a circle as a guide, will be pleased to learn that the guiding circle is, in fact, a circular directrix.

Item Type:Article
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Additional Information:© 2011 Mathematical Association of America.
Issue or Number:2
Record Number:CaltechAUTHORS:20111005-083237390
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Official Citation:Ellipse to Hyperbola: “With This String I Thee Wed”(pp. 83-97) Tom M. Apostol, Mamikon A. Mnatsakanian DOI: 10.4169/math.mag.84.2.083 Stable URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:26585
Deposited By: Ruth Sustaita
Deposited On:05 Oct 2011 16:22
Last Modified:03 Oct 2019 03:16

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