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

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

Rewrite the equation in vertex form.

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

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

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

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

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

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

Divide each term by $- 2$ and simplify.

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

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

Cancel the common factor of $- 2$ .

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Cancel the common factor.

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

Divide $y$ by $1$ .

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

Simplify each term.

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Dividing two negative values results in a positive value.

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

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

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Factor $2$ out of $4 x$ .

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

Move the negative one from the denominator of $\frac{2 x}{- 1}$ .

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

Rewrite $- 1 \cdot (2 x)$ as $- (2 x)$ .

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

Multiply $2$ by $- 1$ .

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

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

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

$a = \frac{1}{2}, b = - 2, c = 0$

Consider the vertex form of a parabola.

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

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

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

Simplify the right side.

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Cancel the common factor of $2$ .

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Cancel the common factor.

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

Rewrite the expression.

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

Multiply the numerator by the reciprocal of the denominator.

$d = - (1 \cdot 2)$

Multiply $2$ by $1$ .

$d = - 1 \cdot 2$

Multiply $- 1$ by $2$ .

$d = - 2$

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

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Simplify each term.

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

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

Combine $4$ and $\frac{1}{2}$ .

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

Divide $4$ by $2$ .

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

Divide $4$ by $2$ .

$e = 0 - 1 \cdot 2$

Multiply $- 1$ by $2$ .

$e = 0 - 2$

Subtract $2$ from $0$ .

$e = - 2$

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

$\frac{1}{2} \cdot {(x - 2)}^{2} - 2$

Set $y$ equal to the new right side.

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

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

$a = \frac{1}{2}$

$h = 2$

$k = - 2$

Find the vertex $(h, k)$ .

$(2, - 2)$

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

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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.

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

Simplify.

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Combine $4$ and $\frac{1}{2}$ .

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

Divide $4$ by $2$ .

$\frac{1}{2}$

Find the directrix.

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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.

$y = - \frac{5}{2}$

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