Write an equation of a parabola with a vertex at the origin and a directrix at y = 5.

Someone help please !
(PICTURE BELOW)

Question

Write an equation of a parabola with a vertex at the origin and a directrix at y = 5.

Someone help please !
(PICTURE BELOW)

anonymous · Answer

attachment

anonymous · Answer

@satellite73 , @terenzreignz

anonymous · Answer

|dw:1361289261716:dw|

anonymous · Answer

you know it faces down so it must look like $y=-ax^2$

anonymous · Answer

i know it's a negative answer

anonymous · Answer

you can write it as $-4py=x^2$ where $p=5$ so you get $-20y=x^2$ or 
$$y=-\frac{1}{20}x^2$$

anonymous · Answer

i got it. Thank you. my answer is c correct. i had it right

terenzreignz · Answer

In general, @Kayy_Drizzyy 
If your vertex is at a point (h , k)
Then your parabola would have the equation
(x-h)² =(+/-) 4p(y-k)
depending on where your parabola faces.

anonymous · Answer

good
did you get the last one?

anonymous · Answer

no. I'm skipping that for last @satellite73

anonymous · Answer

the one with no right answers?

anonymous · Answer

ok