Precalculus Examples

$f (x) = - 3 x^{2} + 6 x + 5$

Step 1

Find the properties of the given parabola.

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Step 1.1

Rewrite the equation in vertex form.

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Step 1.1.1

Complete the square for $- 3 x^{2} + 6 x + 5$ .

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Step 1.1.1.1

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

$a = - 3$

$b = 6$

$c = 5$

Step 1.1.1.2

Consider the vertex form of a parabola.

$a {(x + d)}^{2} + e$

Step 1.1.1.3

Find the value of $d$ using the formula $d = \frac{b}{2 a}$ .

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Step 1.1.1.3.1

Substitute the values of $a$ and $b$ into the formula $d = \frac{b}{2 a}$ .

$d = \frac{6}{2 \cdot - 3}$

Step 1.1.1.3.2

Simplify the right side.

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Step 1.1.1.3.2.1

Cancel the common factor of $6$ and $2$ .

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Step 1.1.1.3.2.1.1

Factor $2$ out of $6$ .

$d = \frac{2 \cdot 3}{2 \cdot - 3}$

Step 1.1.1.3.2.1.2

Cancel the common factors.

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Step 1.1.1.3.2.1.2.1

Factor $2$ out of $2 \cdot - 3$ .

$d = \frac{2 \cdot 3}{2 (- 3)}$

Step 1.1.1.3.2.1.2.2

Cancel the common factor.

$d = \frac{2 \cdot 3}{2 \cdot - 3}$

Step 1.1.1.3.2.1.2.3

Rewrite the expression.

$d = \frac{3}{- 3}$

Step 1.1.1.3.2.2

Cancel the common factor of $3$ and $- 3$ .

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Step 1.1.1.3.2.2.1

Factor $3$ out of $3$ .

$d = \frac{3 (1)}{- 3}$

Step 1.1.1.3.2.2.2

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

$d = - 1 \cdot 1$

Step 1.1.1.3.2.3

Multiply $- 1$ by $1$ .

$d = - 1$

Step 1.1.1.4

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

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Step 1.1.1.4.1

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

$e = 5 - \frac{6^{2}}{4 \cdot - 3}$

Step 1.1.1.4.2

Simplify the right side.

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Step 1.1.1.4.2.1

Simplify each term.

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Step 1.1.1.4.2.1.1

Raise $6$ to the power of $2$ .

$e = 5 - \frac{36}{4 \cdot - 3}$

Step 1.1.1.4.2.1.2

Multiply $4$ by $- 3$ .

$e = 5 - \frac{36}{- 12}$

Step 1.1.1.4.2.1.3

Divide $36$ by $- 12$ .

$e = 5 - - 3$

Step 1.1.1.4.2.1.4

Multiply $- 1$ by $- 3$ .

$e = 5 + 3$

Step 1.1.1.4.2.2

Add $5$ and $3$ .

$e = 8$

Step 1.1.1.5

Substitute the values of $a$ , $d$ , and $e$ into the vertex form $- 3 {(x - 1)}^{2} + 8$ .

$- 3 {(x - 1)}^{2} + 8$

Step 1.1.2

Set $y$ equal to the new right side.

$y = - 3 {(x - 1)}^{2} + 8$

Step 1.2

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

$a = - 3$

$h = 1$

$k = 8$

Step 1.3

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

Opens Down

Step 1.4

Find the vertex $(h, k)$ .

$(1, 8)$

Step 1.5

Find $p$ , the distance from the vertex to the focus.

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Step 1.5.1

Find the distance from the vertex to a focus of the parabola by using the following formula.

$\frac{1}{4 a}$

Step 1.5.2

Substitute the value of $a$ into the formula.

$\frac{1}{4 \cdot - 3}$

Step 1.5.3

Simplify.

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Step 1.5.3.1

Multiply $4$ by $- 3$ .

$\frac{1}{- 12}$

Step 1.5.3.2

Move the negative in front of the fraction.

$- \frac{1}{12}$

Step 1.6

Find the focus.

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Step 1.6.1

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

$(h, k + p)$

Step 1.6.2

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

$(1, \frac{95}{12})$

Step 1.7

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

$x = 1$

Step 1.8

Find the directrix.

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Step 1.8.1

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.

$y = k - p$

Step 1.8.2

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

$y = \frac{97}{12}$

Step 1.9

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

Direction: Opens Down

Vertex: $(1, 8)$

Focus: $(1, \frac{95}{12})$

Axis of Symmetry: $x = 1$

Directrix: $y = \frac{97}{12}$

Direction: Opens Down

Vertex: $(1, 8)$

Focus: $(1, \frac{95}{12})$

Axis of Symmetry: $x = 1$

Directrix: $y = \frac{97}{12}$

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.

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Step 2.1

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

$f (0) = - 3 {(0)}^{2} + 6 (0) + 5$

Step 2.2

Simplify the result.

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Step 2.2.1

Simplify each term.

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Step 2.2.1.1

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

$f (0) = - 3 \cdot 0 + 6 (0) + 5$

Step 2.2.1.2

Multiply $- 3$ by $0$ .

$f (0) = 0 + 6 (0) + 5$

Step 2.2.1.3

Multiply $6$ by $0$ .

$f (0) = 0 + 0 + 5$

Step 2.2.2

Simplify by adding numbers.

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Step 2.2.2.1

Add $0$ and $0$ .

$f (0) = 0 + 5$

Step 2.2.2.2

Add $0$ and $5$ .

$f (0) = 5$

Step 2.2.3

The final answer is $5$ .

$5$

Step 2.3

The $y$ value at $x = 0$ is $5$ .

$y = 5$

Step 2.4

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

$f (- 1) = - 3 {(- 1)}^{2} + 6 (- 1) + 5$

Step 2.5

Simplify the result.

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Step 2.5.1

Simplify each term.

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Step 2.5.1.1

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

$f (- 1) = - 3 \cdot 1 + 6 (- 1) + 5$

Step 2.5.1.2

Multiply $- 3$ by $1$ .

$f (- 1) = - 3 + 6 (- 1) + 5$

Step 2.5.1.3

Multiply $6$ by $- 1$ .

$f (- 1) = - 3 - 6 + 5$

Step 2.5.2

Simplify by adding and subtracting.

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Step 2.5.2.1

Subtract $6$ from $- 3$ .

$f (- 1) = - 9 + 5$

Step 2.5.2.2

Add $- 9$ and $5$ .

$f (- 1) = - 4$

Step 2.5.3

The final answer is $- 4$ .

$- 4$

Step 2.6

The $y$ value at $x = - 1$ is $- 4$ .

$y = - 4$

Step 2.7

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

$f (2) = - 3 {(2)}^{2} + 6 (2) + 5$

Step 2.8

Simplify the result.

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Step 2.8.1

Simplify each term.

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Step 2.8.1.1

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

$f (2) = - 3 \cdot 4 + 6 (2) + 5$

Step 2.8.1.2

Multiply $- 3$ by $4$ .

$f (2) = - 12 + 6 (2) + 5$

Step 2.8.1.3

Multiply $6$ by $2$ .

$f (2) = - 12 + 12 + 5$

Step 2.8.2

Simplify by adding numbers.

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Step 2.8.2.1

Add $- 12$ and $12$ .

$f (2) = 0 + 5$

Step 2.8.2.2

Add $0$ and $5$ .

$f (2) = 5$

Step 2.8.3

The final answer is $5$ .

$5$

Step 2.9

The $y$ value at $x = 2$ is $5$ .

$y = 5$

Step 2.10

Replace the variable $x$ with $3$ in the expression.

$f (3) = - 3 {(3)}^{2} + 6 (3) + 5$

Step 2.11

Simplify the result.

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Step 2.11.1

Simplify each term.

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Step 2.11.1.1

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

$f (3) = - 3 \cdot 9 + 6 (3) + 5$

Step 2.11.1.2

Multiply $- 3$ by $9$ .

$f (3) = - 27 + 6 (3) + 5$

Step 2.11.1.3

Multiply $6$ by $3$ .

$f (3) = - 27 + 18 + 5$

Step 2.11.2

Simplify by adding numbers.

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Step 2.11.2.1

Add $- 27$ and $18$ .

$f (3) = - 9 + 5$

Step 2.11.2.2

Add $- 9$ and $5$ .

$f (3) = - 4$

Step 2.11.3

The final answer is $- 4$ .

$- 4$

Step 2.12

The $y$ value at $x = 3$ is $- 4$ .

$y = - 4$

Step 2.13

Graph the parabola using its properties and the selected points.

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

Step 3

Graph the parabola using its properties and the selected points.

Direction: Opens Down

Vertex: $(1, 8)$

Focus: $(1, \frac{95}{12})$

Axis of Symmetry: $x = 1$

Directrix: $y = \frac{97}{12}$

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

Step 4