Find the Parabola with Focus (9,0) and Directrix y=-4 (9,0) y=-4

$(9, 0)$ $y = - 4$

Step 1

Since the directrix is vertical, use the equation of a parabola that opens up or down.

${(x - h)}^{2} = 4 p (y - k)$

Step 2

Find the vertex.

The vertex $(h, k)$ is halfway between the directrix and focus. Find the $y$ coordinate of the vertex using the formula $y = \frac{y coordinate of focus + directrix}{2}$ . The $x$ coordinate will be the same as the $x$ coordinate of the focus.

$(9, \frac{0 - 4}{2})$

Simplify the vertex.

Cancel the common factor of $0 - 4$ and $2$ .

Factor $2$ out of $0$ .

$(9, \frac{2 \cdot 0 - 4}{2})$

Factor $2$ out of $- 4$ .

$(9, \frac{2 \cdot 0 + 2 \cdot - 2}{2})$

Factor $2$ out of $2 \cdot 0 + 2 \cdot - 2$ .

$(9, \frac{2 \cdot (0 - 2)}{2})$

Cancel the common factors.

Factor $2$ out of $2$ .

$(9, \frac{2 \cdot (0 - 2)}{2 (1)})$

Cancel the common factor.

$(9, \frac{2 \cdot (0 - 2)}{2 \cdot 1})$

Rewrite the expression.

$(9, \frac{0 - 2}{1})$

Divide $0 - 2$ by $1$ .

$(9, 0 - 2)$

Subtract $2$ from $0$ .

$(9, - 2)$

Step 3

Find the distance from the focus to the vertex.

The distance from the focus to the vertex and from the vertex to the directrix is $| p |$ . Subtract the $y$ coordinate of the vertex from the $y$ coordinate of the focus to find $p$ .

$p = 0 + 2$

Add $0$ and $2$ .

$p = 2$

Step 4

Substitute in the known values for the variables into the equation ${(x - h)}^{2} = 4 p (y - k)$ .

${(x - 9)}^{2} = 4 (2) (y + 2)$

Step 5

Simplify.

${(x - 9)}^{2} = 8 (y + 2)$

Step 6

Find the Parabola with Focus (9,0) and Directrix y=-4 (9,0) y=-4

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