Question

What is the equation of the quadratic graph with a focus of (4, 0) and a directrix of y = 10?

Answer 1

@campbell_st

Answer 2

well there are lots of ways to do this question.

1. picking a point P(x, y) on the parabola and then finding the distance from the focus to P and equating that to the distance from P to the directrix.

2. using a little observation and a standard form of the parabola

\[(x - h)^2 = 4a(y - k)\]

the vertex is (h, k) and a is the focal length.

Answer 3

I saw on another of the same problems where someone mentioned that that half-way point between both y's. In this case they are 0 and 10 which would be 5.

Answer 4

I always choose option 2... it requires drawing a diagram

and you need to know the focal length is the distance between the vertex and focus.
It's also the distance between the vertex and directrix.

so the distance between the focus and directrix is 2a.

so the diagram



ok, so lookin at the diagram the directrix is above the focus, so the parabola is concave down.

can you tell me the distance between the focus and directrix?