Find the Parabola with Focus (0,4) and Directrix y=2 (0,4) y=2

$(0, 4)$ $y = 2$

Step 1

Since the directrix is vertical, use the equation of a parabola that opens up or down.

${(x - h)}^{2} = 4 p (y - k)$

Step 2

Find the vertex.

The vertex $(h, k)$ is halfway between the directrix and focus. Find the $y$ coordinate of the vertex using the formula $y = \frac{y coordinate of focus + directrix}{2}$ . The $x$ coordinate will be the same as the $x$ coordinate of the focus.

$(0, \frac{4 + 2}{2})$

Simplify the vertex.

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

Factor $2$ out of $4$ .

$(0, \frac{2 \cdot 2 + 2}{2})$

Factor $2$ out of $2$ .

$(0, \frac{2 \cdot 2 + 2 \cdot 1}{2})$

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

$(0, \frac{2 \cdot (2 + 1)}{2})$

Cancel the common factors.

Factor $2$ out of $2$ .

$(0, \frac{2 \cdot (2 + 1)}{2 (1)})$

Cancel the common factor.

$(0, \frac{2 \cdot (2 + 1)}{2 \cdot 1})$

Rewrite the expression.

$(0, \frac{2 + 1}{1})$

Divide $2 + 1$ by $1$ .

$(0, 2 + 1)$

Add $2$ and $1$ .

$(0, 3)$

Step 3

Find the distance from the focus to the vertex.

The distance from the focus to the vertex and from the vertex to the directrix is $| p |$ . Subtract the $y$ coordinate of the vertex from the $y$ coordinate of the focus to find $p$ .

$p = 4 - 3$

Subtract $3$ from $4$ .

$p = 1$

Step 4

Substitute in the known values for the variables into the equation ${(x - h)}^{2} = 4 p (y - k)$ .

${(x - 0)}^{2} = 4 (1) (y - 3)$

Step 5

Simplify.

$x^{2} = 4 (y - 3)$

Step 6

Find the Parabola with Focus (0,4) and Directrix y=2 (0,4) y=2

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