We have previously seen how a parabola is defined in terms of parametric equations or alternatively in Cartesian form.
An alternative way to define a parabola is as a locus of points.
Focus and directrix The locus defining a parabola depends on a focus and a directrix.
The focus is a point. For a standard parabola, the focus is located on the x axis a distance $a$ from the origin, that is at the point (a, 0).
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