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2. Focus (geometry) - Wikipedia

   en.wikipedia.org/wiki/Focus_(geometry)

   The ellipse thus generated has its second focus at the center of the directrix circle, and the ellipse lies entirely within the circle. For the parabola, the center of the directrix moves to the point at infinity (see Projective geometry). The directrix "circle" becomes a curve with zero curvature, indistinguishable from a straight line.

3. Parabola - Wikipedia

   en.wikipedia.org/wiki/Parabola

   The definition of a parabola by its focus and directrix can be used for drawing it with help of pins and strings: [9] Choose the focus and the directrix of the parabola. Take a triangle of a set square and prepare a string with length | | (see diagram).

4. Eccentricity (mathematics) - Wikipedia

   en.wikipedia.org/wiki/Eccentricity_(mathematics)

   A family of conic sections of varying eccentricity share a focus point and directrix line, including an ellipse (red, e = 1/2), a parabola (green, e = 1), and a hyperbola (blue, e = 2). The conic of eccentricity 0 in this figure is an infinitesimal circle centered at the focus, and the conic of eccentricity ∞ is an infinitesimally separated ...

5. Conic section - Wikipedia

   en.wikipedia.org/wiki/Conic_section

   Define b by the equations c 2 = a 2 − b 2 for an ellipse and c 2 = a 2 + b 2 for a hyperbola. For a circle, c = 0 so a 2 = b 2, with radius r = a = b. For the parabola, the standard form has the focus on the x-axis at the point (a, 0) and the directrix the line with equation x = −a. In standard form the parabola will always pass through the ...

6. Confocal conic sections - Wikipedia

   en.wikipedia.org/wiki/Confocal_conic_sections

   A parabola has only one focus, and can be considered as a limit curve of a set of ellipses (or a set of hyperbolas), where one focus and one vertex are kept fixed, while the second focus is moved to infinity. If this transformation is performed on each conic in an orthogonal net of confocal ellipses and hyperbolas, the limit is an orthogonal ...

7. File:Parabola with focus and arbitrary line.svg - Wikipedia

   en.wikipedia.org/wiki/File:Parabola_with_focus...

   A parabola, showing arbitrary line (L), focus (F), and vertex (V). Generic points marked for explanation. ... Adapted from this image, which shows L as the directrix ...