Question

Find the standard form of the equation of the parabola with a focus at (-4, 0) and a directrix at x = 4. (2 points)

Answer 1

To find the focus you have to find the center between the to, do you remember the formula's?

Answer 2

check the picture below.

since the focus point is at -4, 0 and the directrix is a vertical line at x = 4, is a horizontal parabola, and it opens to the left-hand-side.

now, notice the distance "p", is just 4 units, however, the parabola is opening to the left-side, thus "p" is negative, then is -4, the vertex is half-way between the focus point and the directrix.


`\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=-4 \end{cases}\implies (y-0)^2=4(-4)(x-0)\implies y^2=-16x \\\\\\ -\cfrac{1}{16}y^2=x`