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I am new in the conics so there are two confusions: 1. How to plot the graph of this parabola equation $y^2-x-2y-1=0$? 2. what would be the equation of parabola when vertex $(3,2)$ and ends of focal chord are $(5,6)$ and $(5, -2)$?


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There is a standard method in algebra for reducing a problem involving a quadratic polynomial with a first-degree term to a problem involving a quadratic polynomial with no first-degree term. – Michael Hardy May 25 '14 at 5:10
For the second problem, since the $ \ x-$ coordinates of the latus rectum (focal chord) are the same, it must be parallel to the $ \ y-$ axis. So this is a "horizontal" parabola, which has the "vertex form" $ \ 4p \ (x - h) \ = \ (y-k)^2 \ $ . Remember that the length of the latus rectum is $ \ 4p \ $ . – RecklessReckoner May 25 '14 at 5:15

Rewrite it: $x = y^2 - 2y - 1 = (y-1)^2 - 2 \to x+2 = (y-1)^2$, and here are the steps to plot the graph:

  1. Plot the vertex $V = (-2,1)$.

  2. Draw a parabola open to the right having the vertex at $V$.

For the equation of the parabola with the added information of the focal chord being $6-(-2)=8$, then $a = \dfrac{1}{f} = \dfrac{1}{8}$. So: $x-3 = a(y-2)^2 = \dfrac{1}{8}\times (y-2)^2$.

In general, if $V = (a,b)$, and the focal chord length is $f$, then the equation of the parabola is:

$x-a = \dfrac{1}{f}\times (y-b)^2$

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Thank you so much LAcarguy.... – zonnie May 25 '14 at 5:12

Problem $1$:

The equation is: $$x=y^2-2y-1$$ Complete the square: $$x=y^2-2y+1-1-1$$ $$x=(y-1)^2-2$$ The parabola will have the vertex at $(-2, \ 1)$. It will be "sideways" (because the $y$ is squared) and will open to the right (because the coefficient of the $y^2$ term is positive). The graph should look something like:

enter image description here

Problem $2$:

This "focal chord" should be the latus rectum. This means that it is parallel to the directrix. How do I know this? The focal chord is a vertical line segment. I know that the directrix will also be vertical. Anyways, this means that the focus will be at the midpoint of the focal chord. The midpoint is: $$\left(\frac{5+5}{2}, \ \frac{6+(-2)}{2}\right)$$ $$\text{Which is }(5, \ 2)$$ I know that the parabola will be a "sideways" one. We know that the vertex is $(3, \ 2)$. This means that the distance between the focus and the vertex is $2$.

Standard equation of sideways parabola: $$4p(x-h)=(y-k)^2$$ Where $p$ is the distance between the focus and vertex. $$4(2)(x-3)=(y-2)^2$$ $$8(x-3)=(y-2)^2$$ $$8x-24=y^2-4y+4$$ $$\color{green}{y^2-8x-4y+28=0}$$ Hope I helped

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Thank you JChau... – zonnie May 25 '14 at 5:10
Gonna do problem 2 now :) – JChau May 25 '14 at 5:11
yes waiting..:) – zonnie May 25 '14 at 5:12
@zonnie I did problem $2$ now – JChau May 25 '14 at 5:26
Thank you so much JChau.. Although I have tried it and done it with the same way as you did.. But the way you solve it is truly the best. :) – zonnie May 25 '14 at 7:13

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