ilovemattc
Mar 17, 2009, 10:51 PM
Graph the following parabola (x-2)^2=(y+1) list the vertex focus and directrix
galactus
Mar 18, 2009, 10:01 AM
Rearranging, we have:
y=(x+2)^{2}-1
That means we have the form y=a(x-h)^{2}+k
where h and k are the vertex coordinates.
The directrix is the same distance from the vertex as the focus is from the vertex.
galactus
Mar 19, 2009, 10:17 AM
Pardon my typo in my first post. That should be y=(x-2)^{2}-1
Note, if we expand we get y=x^{2}-4x+3
The x coordinate of the vertex can also be easily obtained by using x=\frac{-b}{2a}
Then, sub that value back into the quadratic to find its corresponding y coordinate.
Note, you have the form y=ax^{2}+bx+c. This is a concave up parabola.
The distance from the vertex to the focus is p=\frac{1}{4a}.
Consequently, that distance is the same from the vertex to the directrix. You have found the vertex, so use that and find the directrix.
It will just be a horizontal line with coordinates y=some number. Look at the graph. That'll help.