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

Tom M. Apostol and Mamikon A. Mnatsakanian
Mathematics Magazine
Vol. 84, No. 2 (April 2011), pp. 83-97
DOI: 10.4169/math.mag.84.2.083
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Page Count: 15
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Ellipse to Hyperbola: “With This String I Thee Wed”
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Summary 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.

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