Graph y=-x^2-5x+4

$y = - x^{2} - 5 x + 4$

Step 1

Find the properties of the given parabola.

Rewrite the equation in vertex form.

Complete the square for $- x^{2} - 5 x + 4$ .

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

$a = - 1$

$b = - 5$

$c = 4$

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

$d = \frac{- 5}{2 \cdot - 1}$

Simplify the right side.

Multiply $2$ by $- 1$ .

$d = \frac{- 5}{- 2}$

Dividing two negative values results in a positive value.

$d = \frac{5}{2}$

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

$e = 4 - \frac{{(- 5)}^{2}}{4 \cdot - 1}$

Simplify the right side.

Simplify each term.

Raise $- 5$ to the power of $2$ .

$e = 4 - \frac{25}{4 \cdot - 1}$

Multiply $4$ by $- 1$ .

$e = 4 - \frac{25}{- 4}$

Move the negative in front of the fraction.

$e = 4 - - \frac{25}{4}$

Multiply $- - \frac{25}{4}$ .

Multiply $- 1$ by $- 1$ .

$e = 4 + 1 (\frac{25}{4})$

Multiply $\frac{25}{4}$ by $1$ .

$e = 4 + \frac{25}{4}$

To write $4$ as a fraction with a common denominator, multiply by $\frac{4}{4}$ .

$e = 4 \cdot \frac{4}{4} + \frac{25}{4}$

Combine $4$ and $\frac{4}{4}$ .

$e = \frac{4 \cdot 4}{4} + \frac{25}{4}$

Combine the numerators over the common denominator.

$e = \frac{4 \cdot 4 + 25}{4}$

Simplify the numerator.

Multiply $4$ by $4$ .

$e = \frac{16 + 25}{4}$

Add $16$ and $25$ .

$e = \frac{41}{4}$

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- {(x + \frac{5}{2})}^{2} + \frac{41}{4}$ .

$- {(x + \frac{5}{2})}^{2} + \frac{41}{4}$

Set $y$ equal to the new right side.

$y = - {(x + \frac{5}{2})}^{2} + \frac{41}{4}$

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

$a = - 1$

$h = - \frac{5}{2}$

$k = \frac{41}{4}$

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

Opens Down

Find the vertex $(h, k)$ .

$(- \frac{5}{2}, \frac{41}{4})$

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.

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

Cancel the common factor of $1$ and $- 1$ .

Rewrite $1$ as $- 1 (- 1)$ .

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

Move the negative in front of the fraction.

$- \frac{1}{4}$

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.

$(- \frac{5}{2}, 10)$

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

$x = - \frac{5}{2}$

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.

$y = \frac{21}{2}$

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

Direction: Opens Down

Vertex: $(- \frac{5}{2}, \frac{41}{4})$

Focus: $(- \frac{5}{2}, 10)$

Axis of Symmetry: $x = - \frac{5}{2}$

Directrix: $y = \frac{21}{2}$

Direction: Opens Down

Vertex: $(- \frac{5}{2}, \frac{41}{4})$

Focus: $(- \frac{5}{2}, 10)$

Axis of Symmetry: $x = - \frac{5}{2}$

Directrix: $y = \frac{21}{2}$

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 $- 3$ in the expression.

$f (- 3) = - {(- 3)}^{2} - 5 \cdot - 3 + 4$

Simplify the result.

Simplify each term.

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

$f (- 3) = - 1 \cdot 9 - 5 \cdot - 3 + 4$

Multiply $- 1$ by $9$ .

$f (- 3) = - 9 - 5 \cdot - 3 + 4$

Multiply $- 5$ by $- 3$ .

$f (- 3) = - 9 + 15 + 4$

Simplify by adding numbers.

Add $- 9$ and $15$ .

$f (- 3) = 6 + 4$

Add $6$ and $4$ .

$f (- 3) = 10$

The final answer is $10$ .

$10$

The $y$ value at $x = - 3$ is $10$ .

$y = 10$

Replace the variable $x$ with $- 4$ in the expression.

$f (- 4) = - {(- 4)}^{2} - 5 \cdot - 4 + 4$

Simplify the result.

Simplify each term.

Raise $- 4$ to the power of $2$ .

$f (- 4) = - 1 \cdot 16 - 5 \cdot - 4 + 4$

Multiply $- 1$ by $16$ .

$f (- 4) = - 16 - 5 \cdot - 4 + 4$

Multiply $- 5$ by $- 4$ .

$f (- 4) = - 16 + 20 + 4$

Simplify by adding numbers.

Add $- 16$ and $20$ .

$f (- 4) = 4 + 4$

Add $4$ and $4$ .

$f (- 4) = 8$

The final answer is $8$ .

$8$

The $y$ value at $x = - 4$ is $8$ .

$y = 8$

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

$f (- 1) = - {(- 1)}^{2} - 5 \cdot - 1 + 4$

Simplify the result.

Simplify each term.

Multiply $- 1$ by ${(- 1)}^{2}$ by adding the exponents.

Multiply $- 1$ by ${(- 1)}^{2}$ .

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

$f (- 1) = (- 1) {(- 1)}^{2} - 5 \cdot - 1 + 4$

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

$f (- 1) = {(- 1)}^{1 + 2} - 5 \cdot - 1 + 4$

Add $1$ and $2$ .

$f (- 1) = {(- 1)}^{3} - 5 \cdot - 1 + 4$

Raise $- 1$ to the power of $3$ .

$f (- 1) = - 1 - 5 \cdot - 1 + 4$

Multiply $- 5$ by $- 1$ .

$f (- 1) = - 1 + 5 + 4$

Simplify by adding numbers.

Add $- 1$ and $5$ .

$f (- 1) = 4 + 4$

Add $4$ and $4$ .

$f (- 1) = 8$

The final answer is $8$ .

$8$

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

$y = 8$

Replace the variable $x$ with $0$ in the expression.

$f (0) = - {(0)}^{2} - 5 \cdot 0 + 4$

Simplify the result.

Simplify each term.

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

$f (0) = - 0 - 5 \cdot 0 + 4$

Multiply $- 1$ by $0$ .

$f (0) = 0 - 5 \cdot 0 + 4$

Multiply $- 5$ by $0$ .

$f (0) = 0 + 0 + 4$

Simplify by adding numbers.

Add $0$ and $0$ .

$f (0) = 0 + 4$

Add $0$ and $4$ .

$f (0) = 4$

The final answer is $4$ .

$4$

The $y$ value at $x = 0$ is $4$ .

$y = 4$

Graph the parabola using its properties and the selected points.

$\begin{array}{c} x & y \\ - 4 & 8 \\ - 3 & 10 \\ - \frac{5}{2} & \frac{41}{4} \\ - 1 & 8 \\ 0 & 4 \end{array}$

Step 3

Graph the parabola using its properties and the selected points.

Direction: Opens Down

Vertex: $(- \frac{5}{2}, \frac{41}{4})$

Focus: $(- \frac{5}{2}, 10)$

Axis of Symmetry: $x = - \frac{5}{2}$

Directrix: $y = \frac{21}{2}$

$\begin{array}{c} x & y \\ - 4 & 8 \\ - 3 & 10 \\ - \frac{5}{2} & \frac{41}{4} \\ - 1 & 8 \\ 0 & 4 \end{array}$

Step 4

Graph y=-x^2-5x+4

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