Derive the equation of the parabola with a focus at (4, -7) and a directrix of y = -15. Put the equation in standard form

Question

anonymous · Answer

a.  f(x) = 1/16  x2 - 8x + 11
b.  f(x) = 1/16  x2 - 8x - 10
d.  f(x) = 1/16  x2 - 1/2 x - 10

anonymous · Answer

C was wrong

campbell_st · Answer

well you need to find the vertex. 

but before that you need to find the focal length

the distance between the directrix and focus is equal to 2 times the focal length. 

so what do you think the focal length would be..?

anonymous · Answer

I'm totally lost! The practice problems don't make sense at all:(

anonymous · Answer

And in the lesson it didn't say anything about needing the vertex or focal length I don't think

campbell_st · Answer

ok... 
|dw:1385058491849:dw|

the vertex in the line x = 4 and located half way between  y = -7 and y = -15

the total distance between y = -7( the focus) and y = -15( the directrix) is a length which is double the focal length