Find the Directrix x^2+8x+8y=0

$x^{2} + 8 x + 8 y = 0$

Step 1

Rewrite the equation in vertex form.

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

Move all terms not containing $y$ to the right side of the equation.

Subtract $x^{2}$ from both sides of the equation.

$8 x + 8 y = - x^{2}$

Subtract $8 x$ from both sides of the equation.

$8 y = - x^{2} - 8 x$

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

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

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

Simplify the left side.

Cancel the common factor of $8$ .

Cancel the common factor.

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

Divide $y$ by $1$ .

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

Simplify the right side.

Simplify each term.

Move the negative in front of the fraction.

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

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

Factor $8$ out of $- 8 x$ .

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

Cancel the common factors.

Factor $8$ out of $8$ .

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

Cancel the common factor.

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

Rewrite the expression.

$y = - \frac{x^{2}}{8} + \frac{- x}{1}$

Divide $- x$ by $1$ .

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

Complete the square for $- \frac{x^{2}}{8} - x$ .

Use the form $a x^{2} + b x + c$ , to find the values of $a$ , $b$ , and $c$ .

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

$b = - 1$

$c = 0$

Consider the vertex form of a parabola.

$a {(x + d)}^{2} + e$

Find the value of $d$ using the formula $d = \frac{b}{2 a}$ .

Substitute the values of $a$ and $b$ into the formula $d = \frac{b}{2 a}$ .

$d = \frac{- 1}{2 (- \frac{1}{8})}$

Simplify the right side.

Dividing two negative values results in a positive value.

$d = \frac{1}{2 (\frac{1}{8})}$

Combine $2$ and $\frac{1}{8}$ .

$d = \frac{1}{\frac{2}{8}}$

Cancel the common factor of $2$ and $8$ .

Factor $2$ out of $2$ .

$d = \frac{1}{\frac{2 (1)}{8}}$

Cancel the common factors.

Factor $2$ out of $8$ .

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

Cancel the common factor.

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

Rewrite the expression.

$d = \frac{1}{\frac{1}{4}}$

Multiply the numerator by the reciprocal of the denominator.

$d = 1 \cdot 4$

Multiply $4$ by $1$ .

$d = 4$

Find the value of $e$ using the formula $e = c - \frac{b^{2}}{4 a}$ .

Substitute the values of $c$ , $b$ and $a$ into the formula $e = c - \frac{b^{2}}{4 a}$ .

$e = 0 - \frac{{(- 1)}^{2}}{4 (- \frac{1}{8})}$

Simplify the right side.

Simplify each term.

Raise $- 1$ to the power of $2$ .

$e = 0 - \frac{1}{4 (- \frac{1}{8})}$

Simplify the denominator.

Multiply $- 1$ by $4$ .

$e = 0 - \frac{1}{- 4 (\frac{1}{8})}$

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

$e = 0 - \frac{1}{\frac{- 4}{8}}$

Reduce the expression by cancelling the common factors.

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

Factor $4$ out of $- 4$ .

$e = 0 - \frac{1}{\frac{4 (- 1)}{8}}$

Cancel the common factors.

Factor $4$ out of $8$ .

$e = 0 - \frac{1}{\frac{4 \cdot - 1}{4 \cdot 2}}$

Cancel the common factor.

$e = 0 - \frac{1}{\frac{4 \cdot - 1}{4 \cdot 2}}$

Rewrite the expression.

$e = 0 - \frac{1}{\frac{- 1}{2}}$

Move the negative in front of the fraction.

$e = 0 - \frac{1}{- \frac{1}{2}}$

Cancel the common factor of $1$ and $- 1$ .

Rewrite $1$ as $- 1 (- 1)$ .

$e = 0 - \frac{- 1 (- 1)}{- \frac{1}{2}}$

Move the negative in front of the fraction.

$e = 0 - - \frac{1}{\frac{1}{2}}$

Multiply the numerator by the reciprocal of the denominator.

$e = 0 - - (1 \cdot 2)$

Multiply $- - (1 \cdot 2)$ .

Multiply $2$ by $1$ .

$e = 0 - (- 1 \cdot 2)$

Multiply $- 1$ by $2$ .

$e = 0 - - 2$

Multiply $- 1$ by $- 2$ .

$e = 0 + 2$

Add $0$ and $2$ .

$e = 2$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- \frac{1}{8} {(x + 4)}^{2} + 2$ .

$- \frac{1}{8} {(x + 4)}^{2} + 2$

Set $y$ equal to the new right side.

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

Step 2

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

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

$h = - 4$

$k = 2$

Step 3

Find the vertex $(h, k)$ .

$(- 4, 2)$

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.

Cancel the common factor of $1$ and $- 1$ .

Rewrite $1$ as $- 1 (- 1)$ .

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

Move the negative in front of the fraction.

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

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 $- (1 \cdot 2)$ .

Multiply $2$ by $1$ .

$- 1 \cdot 2$

Multiply $- 1$ by $2$ .

$- 2$

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.

$y = 4$

Step 6

Find the Directrix x^2+8x+8y=0

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