Directrix:
If the vertex of the parabola is at (0, 0) and the focus is (0, -4) which is 4 units down then the directrix will be the line 4 units UP.
Answer: y = 4
Equation of parabola:
[tex]y = \frac{1}{4p}x^{2}[/tex] ; where p is the distance from the vertex to the focus.
As stated above, the focus is 4 units down, so p = -4
[tex]y = \frac{1}{4(-4)}x^{2}[/tex]
[tex]y = \frac{1}{-16}x^{2}[/tex]
Answer: [tex]y = -\frac{1}{16}x^{2}[/tex]
