Find the Directrix -2x^2-4x+y+70=0

$- 2 x^{2} - 4 x + y + 70 = 0$

Step 1

Rewrite the equation in vertex form.

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

Add $2 x^{2}$ to both sides of the equation.

$- 4 x + y + 70 = 2 x^{2}$

Add $4 x$ to both sides of the equation.

$y + 70 = 2 x^{2} + 4 x$

Subtract $70$ from both sides of the equation.

$y = 2 x^{2} + 4 x - 70$

Complete the square for $2 x^{2} + 4 x - 70$ .

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

$a = 2$

$b = 4$

$c = - 70$

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 $d = \frac{b}{2 a}$ .

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

Simplify the right side.

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

Factor $2$ out of $4$ .

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

Cancel the common factors.

Factor $2$ out of $2 \cdot 2$ .

$d = \frac{2 \cdot 2}{2 (2)}$

Cancel the common factor.

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

Rewrite the expression.

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

Cancel the common factor of $2$ .

Cancel the common factor.

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

Rewrite the expression.

$d = 1$

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 $e = c - \frac{b^{2}}{4 a}$ .

$e = - 70 - \frac{4^{2}}{4 \cdot 2}$

Simplify the right side.

Simplify each term.

Cancel the common factor of $4^{2}$ and $4$ .

Factor $4$ out of $4^{2}$ .

$e = - 70 - \frac{4 \cdot 4}{4 \cdot 2}$

Cancel the common factors.

Factor $4$ out of $4 \cdot 2$ .

$e = - 70 - \frac{4 \cdot 4}{4 (2)}$

Cancel the common factor.

$e = - 70 - \frac{4 \cdot 4}{4 \cdot 2}$

Rewrite the expression.

$e = - 70 - \frac{4}{2}$

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

Factor $2$ out of $4$ .

$e = - 70 - \frac{2 \cdot 2}{2}$

Cancel the common factors.

Factor $2$ out of $2$ .

$e = - 70 - \frac{2 \cdot 2}{2 (1)}$

Cancel the common factor.

$e = - 70 - \frac{2 \cdot 2}{2 \cdot 1}$

Rewrite the expression.

$e = - 70 - \frac{2}{1}$

Divide $2$ by $1$ .

$e = - 70 - 1 \cdot 2$

Multiply $- 1$ by $2$ .

$e = - 70 - 2$

Subtract $2$ from $- 70$ .

$e = - 72$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $2 {(x + 1)}^{2} - 72$ .

$2 {(x + 1)}^{2} - 72$

Set $y$ equal to the new right side.

$y = 2 {(x + 1)}^{2} - 72$

Step 2

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

$a = 2$

$h = - 1$

$k = - 72$

Step 3

Find the vertex $(h, k)$ .

$(- 1, - 72)$

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.

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

Multiply $4$ by $2$ .

$\frac{1}{8}$

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.

$y = - \frac{577}{8}$

Step 6

Find the Directrix -2x^2-4x+y+70=0

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