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Definition: The directrix of a conic section is the line which, together with the point known as the focus, serves to define a conic section as the locus of points whose distance from the focus is proportional to the horizontal distance from the directrix, with r being the constant of proportionality. If the ratio r=1, the conic is a parabola, if r<1, it is an ellipse, and if r>1, it is a hyperbola (Hilbert and Cohn-Vossen 1999, p. 27).