SOLUTION: I have to find the coordinates of the focus, vertex, axis of symmetry, and directrix. x=1/4y^2-1/2y-3 I got stuck after I factored out the 1/4. Thank you in advance!

Algebra ->  Quadratic-relations-and-conic-sections -> SOLUTION: I have to find the coordinates of the focus, vertex, axis of symmetry, and directrix. x=1/4y^2-1/2y-3 I got stuck after I factored out the 1/4. Thank you in advance!      Log On


   



Question 911490: I have to find the coordinates of the focus, vertex, axis of symmetry, and directrix.
x=1/4y^2-1/2y-3
I got stuck after I factored out the 1/4. Thank you in advance!

Answer by AnlytcPhil(1806) About Me  (Show Source):
You can put this solution on YOUR website!
x%22%22=%22%22expr%281%2F4%29y%5E2-expr%281%2F2%29y-3
The easiest way when there are fractions is to clear of fractions
first.  Multiply through by 4


4x%22%22=%22%22y%5E2-2y-12

Add 12 to both sides

4x%2B12%22%22=%22%22y%5E2-2y

Complete the square on the right:

Multiply the coefficient of y by 1%2F2: 2%2A%281%2F2%29 = -1
Square that result %28-1%29%5E2=%22%22%2B1
Add +1 to both sides

4x%2B13%22%22=%22%22y%5E2-2y%2B1

Factor the right side:

       y%5E2-2y%2B1=%28y-1%29%28y-1%29=%28y-1%29%5E2

4x%2B13%22%22=%22%22%28y-1%29%5E2

Factor 4 out of the left sides:

4%28x%2B13%2F4%29%22%22=%22%22%28y-1%29%5E2

Multiply both sides by 1%2F4

x%2B13%2F4%22%22=%22%22expr%281%2F4%29%28y-1%29%5E2

Compare to 

x-h%22%22=%22%224p%28y-k%29%5E2

and we have vertex = (h,k) = (-13%2F4,1), 4p=1%2F4, p=1%2F16

It is a parabola with horizontal axis of symmetry, the green line below
whose equation is y=1, since 1 is the y-coordinate of the vertex.



To find the focus we know it is p=1%2F16 of a unit right of the vertex,
so we add 1%2F16 to the x-coordinate of the vertex:

-13%2F4%2B1%2F16=-52%2F16%2B1%2F16=-51%2F16 and the y-coordinate of the focus is
the same as the y-coordinate of the vertex, or 1.

So the focus is the point (-51%2F16,1)

The directrix is a blue line p=1%2F16 of a unit left of the vertex.
-13%2F4-1%2F16=-52%2F16-1%2F16=-53%2F16

So the equation of the directrix is x=-53%2F16

The focus is the point (-51%2F16,1) marked just right of the vertex 
and the directrix is the blue line x=-53%2F16:



Edwin