Find the Axis of Symmetry y=-2x^2+8x-18

$y = - 2 x^{2} + 8 x - 18$

Step 1

Rewrite the equation in vertex form.

Complete the square for $- 2 x^{2} + 8 x - 18$ .

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

$a = - 2$

$b = 8$

$c = - 18$

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

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

Simplify the right side.

Cancel the common factor of $8$ and $2$ .

Factor $2$ out of $8$ .

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

Cancel the common factors.

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

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

Cancel the common factor.

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

Rewrite the expression.

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

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

Factor $2$ out of $4$ .

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

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

$d = - 1 \cdot 2$

Multiply $- 1$ by $2$ .

$d = - 2$

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

Substitute the values of $c$ , $b$ and $a$ into the formula $e = c - \frac{b^{2}}{4 a}$ .

$e = - 18 - \frac{8^{2}}{4 \cdot - 2}$

Simplify the right side.

Simplify each term.

Raise $8$ to the power of $2$ .

$e = - 18 - \frac{64}{4 \cdot - 2}$

Multiply $4$ by $- 2$ .

$e = - 18 - \frac{64}{- 8}$

Divide $64$ by $- 8$ .

$e = - 18 - - 8$

Multiply $- 1$ by $- 8$ .

$e = - 18 + 8$

Add $- 18$ and $8$ .

$e = - 10$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- 2 {(x - 2)}^{2} - 10$ .

$- 2 {(x - 2)}^{2} - 10$

Set $y$ equal to the new right side.

$y = - 2 {(x - 2)}^{2} - 10$

Step 2

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

$a = - 2$

$h = 2$

$k = - 10$

Step 3

Since the value of $a$ is negative, the parabola opens down.

Opens Down

Step 4

Find the vertex $(h, k)$ .

$(2, - 10)$

Step 5

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.

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

Simplify.

Multiply $4$ by $- 2$ .

$\frac{1}{- 8}$

Move the negative in front of the fraction.

$- \frac{1}{8}$

Step 6

Find the focus.

The focus of a parabola can be found by adding $p$ to the y-coordinate $k$ if the parabola opens up or down.

$(h, k + p)$

Substitute the known values of $h$ , $p$ , and $k$ into the formula and simplify.

$(2, - \frac{81}{8})$

Step 7

Find the axis of symmetry by finding the line that passes through the vertex and the focus.

$x = 2$

Step 8

Find the Axis of Symmetry y=-2x^2+8x-18

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