Focus of a parabola

[Mentallic]

1. The problem statement, all variables and given/known data
This problem is from the Australian HSC mathematics extension 2 exam. Q6ciii)

It states:

Find the focus, S, of the parabola y^2=r^2+c^2-2cx where r and c are constants.


3. The attempt at a solution
I couldn't figure out how to convert this into the parabola focus form (which, from the top of my head) might be (x-x_o)^2=4a(y-y_o) for the focus S(x_o,y_o+a)

How is this done?

[lanedance]

notice the form of x & y are reversed, so you will be looking the equation in the form

(y-y_o)^2=4a(x-x_o)

with cordinates in the focus changed as well

note that the co-efficient of y is zero, which implies y_o must be zero in the above... see how you go form here

[Mark44]

In your formula, the parabola's vertex is at (x0, y0) and it opens upward if a > 0. The parabola you're working with opens to the left if c > 0, and to the right if c < 0.

Can you put your parabola in the form (y - y)2 = 4a(x - x0)?

[Mentallic]

Oh ok so r^2+c^2-2cx\equiv 4a(x-x_o)

LHS=-2c(x-b)=-2cx+2cb

therefore 2cb=r^2+c^2

then b=\frac{r^2+c^2}{2c}

Finally, y^2=-2c(x-\frac{r^2+c^2}{2c})

So then the focus is S(\frac{r^2+c^2}{2c}-\frac{c}{2},0)

Is this correct?

edit: simplified, S(\frac{r^2}{2c},0)

[lanedance]

i haven't checked the original focus defintion, but fr0m what you give I get

y^2=r^2+c^2-2cx

(y - 0)^2=4 \frac{1}{4} (r^2+c^2-2cx)

(y - 0)^2=4 (\frac{-c}{2})(x-\frac{r^2+c^2}{2c})

so
a = \frac{-c}{2}

x_0 = \frac{r^2+c^2}{2c}

y_0 = 0

then
focus = ((x_0 + a), y_))

focus = ((\frac{r^2+c^2}{2c} + \frac{-c}{2}), 0)

which look the same