Graph y=-2x^2-4

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

Step 1

Find the properties of the given parabola.

Rewrite the equation in vertex form.

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

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

$a = - 2$

$b = 0$

$c = - 4$

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

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

Simplify the right side.

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

Factor $2$ out of $0$ .

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

Cancel the common factors.

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

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

Cancel the common factor.

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

Rewrite the expression.

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

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

Factor $2$ out of $0$ .

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

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

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

Simplify the right side.

Simplify each term.

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

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

Multiply $4$ by $- 2$ .

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

Divide $0$ by $- 8$ .

$e = - 4 - 0$

Multiply $- 1$ by $0$ .

$e = - 4 + 0$

Add $- 4$ and $0$ .

$e = - 4$

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

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

Set $y$ equal to the new right side.

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

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

$a = - 2$

$h = 0$

$k = - 4$

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

Opens Down

Find the vertex $(h, k)$ .

$(0, - 4)$

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.

$- \frac{1}{8}$

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.

$(0, - \frac{33}{8})$

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.

$y = - \frac{31}{8}$

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

Direction: Opens Down

Vertex: $(0, - 4)$

Focus: $(0, - \frac{33}{8})$

Axis of Symmetry: $x = 0$

Directrix: $y = - \frac{31}{8}$

Direction: Opens Down

Vertex: $(0, - 4)$

Focus: $(0, - \frac{33}{8})$

Axis of Symmetry: $x = 0$

Directrix: $y = - \frac{31}{8}$

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.

$f (- 1) = - 2 {(- 1)}^{2} - 4$

Simplify the result.

Simplify each term.

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

$f (- 1) = - 2 \cdot 1 - 4$

Multiply $- 2$ by $1$ .

$f (- 1) = - 2 - 4$

Subtract $4$ from $- 2$ .

$f (- 1) = - 6$

The final answer is $- 6$ .

$- 6$

The $y$ value at $x = - 1$ is $- 6$ .

$y = - 6$

Replace the variable $x$ with $- 2$ in the expression.

$f (- 2) = - 2 {(- 2)}^{2} - 4$

Simplify the result.

Simplify each term.

Multiply $- 2$ by ${(- 2)}^{2}$ by adding the exponents.

Multiply $- 2$ by ${(- 2)}^{2}$ .

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

$f (- 2) = (- 2) {(- 2)}^{2} - 4$

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$f (- 2) = {(- 2)}^{1 + 2} - 4$

Add $1$ and $2$ .

$f (- 2) = {(- 2)}^{3} - 4$

Raise $- 2$ to the power of $3$ .

$f (- 2) = - 8 - 4$

Subtract $4$ from $- 8$ .

$f (- 2) = - 12$

The final answer is $- 12$ .

$- 12$

The $y$ value at $x = - 2$ is $- 12$ .

$y = - 12$

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

$f (1) = - 2 {(1)}^{2} - 4$

Simplify the result.

Simplify each term.

One to any power is one.

$f (1) = - 2 \cdot 1 - 4$

Multiply $- 2$ by $1$ .

$f (1) = - 2 - 4$

Subtract $4$ from $- 2$ .

$f (1) = - 6$

The final answer is $- 6$ .

$- 6$

The $y$ value at $x = 1$ is $- 6$ .

$y = - 6$

Replace the variable $x$ with $2$ in the expression.

$f (2) = - 2 {(2)}^{2} - 4$

Simplify the result.

Simplify each term.

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

$f (2) = - 2 \cdot 4 - 4$

Multiply $- 2$ by $4$ .

$f (2) = - 8 - 4$

Subtract $4$ from $- 8$ .

$f (2) = - 12$

The final answer is $- 12$ .

$- 12$

The $y$ value at $x = 2$ is $- 12$ .

$y = - 12$

Graph the parabola using its properties and the selected points.

$\begin{array}{c} x & y \\ - 2 & - 12 \\ - 1 & - 6 \\ 0 & - 4 \\ 1 & - 6 \\ 2 & - 12 \end{array}$

Step 3

Graph the parabola using its properties and the selected points.

Direction: Opens Down

Vertex: $(0, - 4)$

Focus: $(0, - \frac{33}{8})$

Axis of Symmetry: $x = 0$

Directrix: $y = - \frac{31}{8}$

$\begin{array}{c} x & y \\ - 2 & - 12 \\ - 1 & - 6 \\ 0 & - 4 \\ 1 & - 6 \\ 2 & - 12 \end{array}$

Step 4

Graph y=-2x^2-4

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