Find the Axis of Symmetry y=-4x^2+8

$y = - 4 x^{2} + 8$

Step 1

Rewrite the equation in vertex form.

Complete the square for $- 4 x^{2} + 8$ .

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

$a = - 4$

$b = 0$

$c = 8$

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

$d = \frac{0}{2 \cdot - 4}$

Simplify the right side.

Cancel the common factor of $0$ and $2$ .

Factor $2$ out of $0$ .

$d = \frac{2 (0)}{2 \cdot - 4}$

Cancel the common factors.

Factor $2$ out of $2 \cdot - 4$ .

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

Cancel the common factor.

$d = \frac{2 \cdot 0}{2 \cdot - 4}$

Rewrite the expression.

$d = \frac{0}{- 4}$

Cancel the common factor of $0$ and $- 4$ .

Factor $4$ out of $0$ .

$d = \frac{4 (0)}{- 4}$

Move the negative one from the denominator of $\frac{0}{- 1}$ .

$d = - 1 \cdot 0$

Rewrite $- 1 \cdot 0$ as $- 0$ .

$d = - 0$

Multiply $- 1$ by $0$ .

$d = 0$

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

$e = 8 - \frac{0^{2}}{4 \cdot - 4}$

Simplify the right side.

Simplify each term.

Raising $0$ to any positive power yields $0$ .

$e = 8 - \frac{0}{4 \cdot - 4}$

Multiply $4$ by $- 4$ .

$e = 8 - \frac{0}{- 16}$

Divide $0$ by $- 16$ .

$e = 8 - 0$

Multiply $- 1$ by $0$ .

$e = 8 + 0$

Add $8$ and $0$ .

$e = 8$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- 4 {(x + 0)}^{2} + 8$ .

$- 4 {(x + 0)}^{2} + 8$

Set $y$ equal to the new right side.

$y = - 4 {(x + 0)}^{2} + 8$

Step 2

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

$a = - 4$

$h = 0$

$k = 8$

Step 3

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

Opens Down

Step 4

Find the vertex $(h, k)$ .

$(0, 8)$

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.

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

Simplify.

Multiply $4$ by $- 4$ .

$\frac{1}{- 16}$

Move the negative in front of the fraction.

$- \frac{1}{16}$

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.

$(0, \frac{127}{16})$

Step 7

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

$x = 0$

Step 8

Find the Axis of Symmetry y=-4x^2+8

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