Question

Find the standard form of the equation of the parabola with a focus at (3, 0) and a directrix at x = -3.

Answer 1

Check the picture below.

now, keep in mind that the focus point is at 3,0 and the directrix is to the left-hand-side of it, therefore, is a horizontal parabola, and it opens to the right-hand-side, like in the picture.

keep in mind that the vertex is half-way between the focus point and directrix, at a distance "p" from either one, notice the "p" distance is just 3 units, since the parabola is opening to the right, "p" is positive.

$\bf \textit{parabola vertex form with focus point distance}\\\\ \begin{array}{llll} \boxed{(y-{{ k}})^2=4{{ p}}(x-{{ h}})} \\\\ (x-{{ h}})^2=4{{ p}}(y-{{ k}}) \end{array} \qquad \begin{array}{llll} vertex\ ({{ h}},{{ k}})\\\\ {{ p}}=\textit{distance from vertex to }\\ \qquad \textit{ focus or directrix} \end{array}\\\\ -------------------------------\\\\ \begin{cases} h=0\\ k=0\\ p=3 \end{cases}\implies (y-0)^2=4(3)(x-0)\implies y^2=12x\implies \cfrac{1}{12}y^2=x$