Find the Directrix y=x^2-16x+56

$y = x^{2} - 16 x + 56$

Step 1

Rewrite the equation in vertex form.

Complete the square for $x^{2} - 16 x + 56$ .

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

$a = 1$

$b = - 16$

$c = 56$

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

$d = \frac{- 16}{2 \cdot 1}$

Cancel the common factor of $- 16$ and $2$ .

Factor $2$ out of $- 16$ .

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

Cancel the common factors.

Factor $2$ out of $2 \cdot 1$ .

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

Cancel the common factor.

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

Rewrite the expression.

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

Divide $- 8$ by $1$ .

$d = - 8$

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

$e = 56 - \frac{{(- 16)}^{2}}{4 \cdot 1}$

Simplify the right side.

Simplify each term.

Raise $- 16$ to the power of $2$ .

$e = 56 - \frac{256}{4 \cdot 1}$

Multiply $4$ by $1$ .

$e = 56 - \frac{256}{4}$

Divide $256$ by $4$ .

$e = 56 - 1 \cdot 64$

Multiply $- 1$ by $64$ .

$e = 56 - 64$

Subtract $64$ from $56$ .

$e = - 8$

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

${(x - 8)}^{2} - 8$

Set $y$ equal to the new right side.

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

Step 2

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

$a = 1$

$h = 8$

$k = - 8$

Step 3

Find the vertex $(h, k)$ .

$(8, - 8)$

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.

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

Cancel the common factor of $1$ .

Cancel the common factor.

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

Rewrite the expression.

$\frac{1}{4}$

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.

$y = - \frac{33}{4}$

Step 6

Find the Directrix y=x^2-16x+56

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