Find the Directrix y=1/28(x-4)^2-5

$y = \frac{1}{28} {(x - 4)}^{2} - 5$

Step 1

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

$a = \frac{1}{28}$

$h = 4$

$k = - 5$

Step 2

Find the vertex $(h, k)$ .

$(4, - 5)$

Step 3

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.

$\frac{1}{4 \cdot \frac{1}{28}}$

Simplify.

Combine $4$ and $\frac{1}{28}$ .

$\frac{1}{\frac{4}{28}}$

Cancel the common factor of $4$ and $28$ .

Factor $4$ out of $4$ .

$\frac{1}{\frac{4 (1)}{28}}$

Cancel the common factors.

Factor $4$ out of $28$ .

$\frac{1}{\frac{4 \cdot 1}{4 \cdot 7}}$

Cancel the common factor.

$\frac{1}{\frac{4 \cdot 1}{4 \cdot 7}}$

Rewrite the expression.

$\frac{1}{\frac{1}{7}}$

Multiply the numerator by the reciprocal of the denominator.

$1 \cdot 7$

Multiply $7$ by $1$ .

$7$

Step 4

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.

$y = - 12$

Step 5