A point related to the construction and properties of conic sections. Hyperbolas and noncircular ellipses have two distinct foci and two associated conic section directrices, each directrix being perpendicular to the line joining the two foci (Eves 1965, p. 275).
See also
Conic Section Directrix, Eccentricity, Ellipse, Ellipsoid, Focal Parameter, Hyperbola, Hyperboloid, Parabola, Paraboloid, Reflection Property
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References
Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 141-144, 1967.Eves, H. "The Focus-Directrix Property." ยง6.8 in A Survey of Geometry, rev. ed. Boston, MA: Allyn & Bacon, pp. 272-275, 1965.
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Cite this as:
Weisstein, Eric W. "Focus." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Focus.html