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IA idea #6 - Directrix of a parabola

Consider the following property for parabolas. Many students do not know what the directrix of a parabola is - which is a bit of a shame because then they are not aware of the fact that a parabola is the set of points (locus of points) that are equidistant from a line (directrix) and a point not on the line (focus). Here is the outline of a way to start to explore (discover?) this property for parabolas. Choose a...

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