Find the Directrix (x-2)^2=8(y+4)

${(x - 2)}^{2} = 8 (y + 4)$

Step 1

Isolate $y$ to the left side of the equation.

Rewrite the equation as $8 (y + 4) = {(x - 2)}^{2}$ .

$8 (y + 4) = {(x - 2)}^{2}$

Divide each term in $8 (y + 4) = {(x - 2)}^{2}$ by $8$ and simplify.

Divide each term in $8 (y + 4) = {(x - 2)}^{2}$ by $8$ .

$\frac{8 (y + 4)}{8} = \frac{{(x - 2)}^{2}}{8}$

Simplify the left side.

Cancel the common factor of $8$ .

Cancel the common factor.

$\frac{8 (y + 4)}{8} = \frac{{(x - 2)}^{2}}{8}$

Divide $y + 4$ by $1$ .

$y + 4 = \frac{{(x - 2)}^{2}}{8}$

Subtract $4$ from both sides of the equation.

$y = \frac{{(x - 2)}^{2}}{8} - 4$

Reorder terms.

$y = \frac{1}{8} \cdot {(x - 2)}^{2} - 4$

Step 2

Use the vertex form, $y = a {(x - h)}^{2} + k$ , to determine the values of $a$ , $h$ , and $k$ .

$a = \frac{1}{8}$

$h = 2$

$k = - 4$

Step 3

Find the vertex $(h, k)$ .

$(2, - 4)$

Step 4

Find $p$ , the distance from the vertex to the focus.

Find the distance from the vertex to a focus of the parabola by using the following formula.

$\frac{1}{4 a}$

Substitute the value of $a$ into the formula.

$\frac{1}{4 \cdot \frac{1}{8}}$

Simplify.

Combine $4$ and $\frac{1}{8}$ .

$\frac{1}{\frac{4}{8}}$

Cancel the common factor of $4$ and $8$ .

Factor $4$ out of $4$ .

$\frac{1}{\frac{4 (1)}{8}}$

Cancel the common factors.

Factor $4$ out of $8$ .

$\frac{1}{\frac{4 \cdot 1}{4 \cdot 2}}$

Cancel the common factor.

$\frac{1}{\frac{4 \cdot 1}{4 \cdot 2}}$

Rewrite the expression.

$\frac{1}{\frac{1}{2}}$

Multiply the numerator by the reciprocal of the denominator.

$1 \cdot 2$

Multiply $2$ by $1$ .

$2$

Step 5

Find the directrix.

The directrix of a parabola is the horizontal line found by subtracting $p$ from the y-coordinate $k$ of the vertex if the parabola opens up or down.

$y = k - p$

Substitute the known values of $p$ and $k$ into the formula and simplify.

$y = - 6$

Step 6

Find the Directrix (x-2)^2=8(y+4)

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