# focus and directrix

Given point F (=focus), and line (=directrix).
Q is a point equidistant from the point and the line.
Try moving Q

Let , and .
How does changing the parameter affect the parabola?

And how about other points in the plane?
In the applet below, move around point P, and try to notice:
which points are closer to the focus, and which to the directrix?

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