How do you find the standard equation for a parobala when you do not have the directrix? I do have the focus, but the thing is the focus is not a 0,p value, it is -2,5, so I don't see how it could really be a focus. I'm sorry I can't get the graph in this message, but an explanation on finding the standard equation in general will do. Thanks!!!

Question

How do you find the standard equation for a parobala when you do not have the directrix? I do have the focus, but the thing is the focus is not a 0,p value, it is -2,5, so I don't see how it could really be a focus. I'm sorry I can't get the graph in this message, but an explanation on finding the standard equation in general will do.  Thanks!!!

Reiny · Answer

<< ...but the thing is the focus is not a 0,p value, it is -2,5...>>

Is the focus (0,-2.5) or (-2.5,0)
you are also missing some more information.
There has to be at least another point given on the parabola.
Did they perhaps give you the vertex?

Here is a nice webpage that explains a lot about the parabola and its variations of equations to express it.

http://www.algebralab.org/lessons/lesson.aspx?file=Algebra_conics_directrix.xml