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directrix y=9x2
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Solution
y=−136
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Parabola Directrix
A parabola is the locus of points such that the distance to a point (the focus) equals the distance to a line (the directrix)
Parabola standard equation
4p(y−k)=(x−h)2 is the standard equation for an up-down facing parabola with vertex at (h,k), and a focal length |p|
Rewrite y=9x2in the standard form:4·136(y−0)=(x−0)2
(h,k)=(0,0),p=136
Parabola is symmetric around the y-axis and so the directrix is a line parallel to the x-axis, a distance −p from the center (0,0) y-coordinate