What was done wrongly on this problem?

3. A satellite dish has a diameter of 20 inches and a depth of 5 inches at its
center. How far from the vertex is the focus?

Graph the parabola in a coordinate system with
vertex (0, 0).
The vertical parabola has the form y = (1/c)x^2.
5 = 1/c(+/-10)^2
5c = 100
c = 20
The focus is at (0, 20), 20 in. from the vertex.

Question

What was done wrongly on this problem?

3. A satellite dish has a diameter of 20 inches and a depth of 5 inches at its
center. How far from the vertex is the focus?


Graph the parabola in a coordinate system with
vertex (0, 0).
The vertical parabola has the form y = (1/c)x^2.
5 = 1/c(+/-10)^2
5c = 100
c = 20
The focus is at (0, 20), 20 in. from the vertex.

anonymous · Answer

http://i.imgur.com/IgbgZfG.png?1 this is the graph associated with the problem

anonymous · Answer

@jim_thompson5910 @ghazi @robtobey @AccessDenied @Luis_Rivera

anonymous · Answer

I am supposed to find the error in the problem.

anonymous · Answer

@dumbcow @saifoo.khan @zepdrix @wio @timo86m

anonymous · Answer

@Mertsj @mathslover @AravindG

mertsj · Answer

$$x^2=4cy$$
$$y=\frac{1}{4c}x^2$$
$$4c(5)=100$$
$$c=5$$

mertsj · Answer

Focus is at (0,5).  5 units from vertex

anonymous · Answer

Wow, thank you soo much, I finally got a reply, I got it now :)

mertsj · Answer

yw