# Homework Help: Quadritic curves, how can foci = vertice?

1. Aug 2, 2011

### flyingpig

1. The problem statement, all variables and given/known data

[PLAIN]http://img688.imageshack.us/img688/5336/unledaty.jpg [Broken]

3. The attempt at a solution

I underlined a = 3, which doesn't make sense seeing the foci is at (plus/minus3, 3)

How can this be?? Wouldn't that make a straight line?

Last edited by a moderator: May 5, 2017
2. Aug 3, 2011

### eumyang

You're confusing distances and points.

a is the distance between the center and a vertex. The vertices are where the blue curve intersects the focal axis. This distance a = 3.

I use the letter c to indicate the distance between the center and a focus. You can see that while one of the foci is (3, 3), the distance c between the center (which is (0, 0)) and this focus is
$3\sqrt{2}$
, not 3.

3. Aug 3, 2011

### flyingpig

Oh right okay thanks lol