Graph f(x)=-8x^2

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

Step 1

Find the properties of the given parabola.

Rewrite the equation in vertex form.

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

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

$a = - 8$

$b = 0$

$c = 0$

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

$d = \frac{0}{2 \cdot - 8}$

Simplify the right side.

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

Factor $2$ out of $0$ .

$d = \frac{2 (0)}{2 \cdot - 8}$

Cancel the common factors.

Factor $2$ out of $2 \cdot - 8$ .

$d = \frac{2 (0)}{2 (- 8)}$

Cancel the common factor.

$d = \frac{2 \cdot 0}{2 \cdot - 8}$

Rewrite the expression.

$d = \frac{0}{- 8}$

Cancel the common factor of $0$ and $- 8$ .

Factor $8$ out of $0$ .

$d = \frac{8 (0)}{- 8}$

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

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

Simplify the right side.

Simplify each term.

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

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

Multiply $4$ by $- 8$ .

$e = 0 - \frac{0}{- 32}$

Divide $0$ by $- 32$ .

$e = 0 - 0$

Multiply $- 1$ by $0$ .

$e = 0 + 0$

Add $0$ and $0$ .

$e = 0$

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

$- 8 x^{2}$

Set $y$ equal to the new right side.

$y = - 8 x^{2}$

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

$a = - 8$

$h = 0$

$k = 0$

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

Opens Down

Find the vertex $(h, k)$ .

$(0, 0)$

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.

$\frac{1}{4 \cdot - 8}$

Simplify.

Multiply $4$ by $- 8$ .

$\frac{1}{- 32}$

Move the negative in front of the fraction.

$- \frac{1}{32}$

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.

$(0, - \frac{1}{32})$

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

$x = 0$

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.

$y = \frac{1}{32}$

Use the properties of the parabola to analyze and graph the parabola.

Direction: Opens Down

Vertex: $(0, 0)$

Focus: $(0, - \frac{1}{32})$

Axis of Symmetry: $x = 0$

Directrix: $y = \frac{1}{32}$

Direction: Opens Down

Vertex: $(0, 0)$

Focus: $(0, - \frac{1}{32})$

Axis of Symmetry: $x = 0$

Directrix: $y = \frac{1}{32}$

Step 2

Select a few $x$ values, and plug them into the equation to find the corresponding $y$ values. The $x$ values should be selected around the vertex.

Replace the variable $x$ with $- 1$ in the expression.

$f (- 1) = - 8 {(- 1)}^{2}$

Simplify the result.

Raise $- 1$ to the power of $2$ .

$f (- 1) = - 8 \cdot 1$

Multiply $- 8$ by $1$ .

$f (- 1) = - 8$

The final answer is $- 8$ .

$- 8$

The $y$ value at $x = - 1$ is $- 8$ .

$y = - 8$

Replace the variable $x$ with $1$ in the expression.

$f (1) = - 8 {(1)}^{2}$

Simplify the result.

One to any power is one.

$f (1) = - 8 \cdot 1$

Multiply $- 8$ by $1$ .

$f (1) = - 8$

The final answer is $- 8$ .

$- 8$

The $y$ value at $x = 1$ is $- 8$ .

$y = - 8$

Graph the parabola using its properties and the selected points.

$\begin{array}{c} x & y \\ - 1 & - 8 \\ 0 & 0 \\ 1 & - 8 \end{array}$

Step 3

Graph the parabola using its properties and the selected points.

Direction: Opens Down

Vertex: $(0, 0)$

Focus: $(0, - \frac{1}{32})$

Axis of Symmetry: $x = 0$

Directrix: $y = \frac{1}{32}$

$\begin{array}{c} x & y \\ - 1 & - 8 \\ 0 & 0 \\ 1 & - 8 \end{array}$

Step 4

Graph f(x)=-8x^2

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