Algebra Examples

$y = 9 - x^{2}$

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

Reorder

$9$ and

$- x^{2}$ .

$y = - x^{2} + 9$

Step 1.1.2

Complete the square for

$- x^{2} + 9$ .

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Step 1.1.2.1

Use the form

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

$a$ ,

$b$ , and

$c$ .

$a = - 1$

$b = 0$

$c = 9$

Step 1.1.2.2

Consider the vertex form of a parabola.

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

Step 1.1.2.3

Find the value of

$d$ using the formula

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

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Step 1.1.2.3.1

Substitute the values of

$a$ and

$b$ into the formula

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

$d = \frac{0}{2 \cdot - 1}$

Step 1.1.2.3.2

Simplify the right side.

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Step 1.1.2.3.2.1

Cancel the common factor of

$0$ and

$2$ .

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Step 1.1.2.3.2.1.1

Factor

$2$ out of

$0$ .

$d = \frac{2 (0)}{2 \cdot - 1}$

Step 1.1.2.3.2.1.2

Move the negative one from the denominator of

$\frac{0}{- 1}$ .

$d = - 1 \cdot 0$

Step 1.1.2.3.2.2

Rewrite

$- 1 \cdot 0$ as

$- 0$ .

$d = - 0$

Step 1.1.2.3.2.3

Multiply

$- 1$ by

$0$ .

$d = 0$

Step 1.1.2.4

Find the value of

$e$ using the formula

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

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Step 1.1.2.4.1

Substitute the values of

$c$ ,

$b$ and

$a$ into the formula

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

$e = 9 - \frac{0^{2}}{4 \cdot - 1}$

Step 1.1.2.4.2

Simplify the right side.

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Step 1.1.2.4.2.1

Simplify each term.

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Step 1.1.2.4.2.1.1

Raising

$0$ to any positive power yields

$0$ .

$e = 9 - \frac{0}{4 \cdot - 1}$

Step 1.1.2.4.2.1.2

Multiply

$4$ by

$- 1$ .

$e = 9 - \frac{0}{- 4}$

Step 1.1.2.4.2.1.3

Divide

$0$ by

$- 4$ .

$e = 9 - 0$

Step 1.1.2.4.2.1.4

Multiply

$- 1$ by

$0$ .

$e = 9 + 0$

Step 1.1.2.4.2.2

Add

$9$ and

$0$ .

$e = 9$

Step 1.1.2.5

Substitute the values of

$a$ ,

$d$ , and

$e$ into the vertex form

$- {(x + 0)}^{2} + 9$ .

$- {(x + 0)}^{2} + 9$

Step 1.1.3

Set

$y$ equal to the new right side.

$y = - {(x + 0)}^{2} + 9$

Step 1.2

Use the vertex form,

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

$a$ ,

$h$ , and

$k$ .

$a = - 1$

$h = 0$

$k = 9$

Step 1.3

Since the value of

$a$ is negative, the parabola opens down.

Opens Down

Step 1.4

Find the vertex

$(h, k)$ .

$(0, 9)$

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.

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

Step 1.5.3

Cancel the common factor of

$1$ and

$- 1$ .

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Step 1.5.3.1

Rewrite

$1$ as

$- 1 (- 1)$ .

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

Step 1.5.3.2

Move the negative in front of the fraction.

$- \frac{1}{4}$

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.

$(0, \frac{35}{4})$

Step 1.7

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

$x = 0$

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.

$y = \frac{37}{4}$

Step 1.9

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

Direction: Opens Down

Vertex:

$(0, 9)$

Focus:

$(0, \frac{35}{4})$

Axis of Symmetry:

$x = 0$

Directrix:

$y = \frac{37}{4}$

Direction: Opens Down

Vertex:

$(0, 9)$

Focus:

$(0, \frac{35}{4})$

Axis of Symmetry:

$x = 0$

Directrix:

$y = \frac{37}{4}$

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

$- 1$ in the expression.

$f (- 1) = - {(- 1)}^{2} + 9$

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

Multiply

$- 1$ by

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

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Step 2.2.1.1.1

Multiply

$- 1$ by

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

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Step 2.2.1.1.1.1

Raise

$- 1$ to the power of

$1$ .

$f (- 1) = (- 1) {(- 1)}^{2} + 9$

Step 2.2.1.1.1.2

Use the power rule

$a^{m} a^{n} = a^{m + n}$ to combine exponents.

$f (- 1) = {(- 1)}^{1 + 2} + 9$

Step 2.2.1.1.2

Add

$1$ and

$2$ .

$f (- 1) = {(- 1)}^{3} + 9$

Step 2.2.1.2

Raise

$- 1$ to the power of

$3$ .

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

Step 2.2.2

Add

$- 1$ and

$9$ .

$f (- 1) = 8$

Step 2.2.3

The final answer is

$8$ .

$8$

Step 2.3

The

$y$ value at

$x = - 1$ is

$8$ .

$y = 8$

Step 2.4

Replace the variable

$x$ with

$- 2$ in the expression.

$f (- 2) = - {(- 2)}^{2} + 9$

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

$- 2$ to the power of

$2$ .

$f (- 2) = - 1 \cdot 4 + 9$

Step 2.5.1.2

Multiply

$- 1$ by

$4$ .

$f (- 2) = - 4 + 9$

Step 2.5.2

Add

$- 4$ and

$9$ .

$f (- 2) = 5$

Step 2.5.3

The final answer is

$5$ .

$5$

Step 2.6

The

$y$ value at

$x = - 2$ is

$5$ .

$y = 5$

Step 2.7

Replace the variable

$x$ with

$1$ in the expression.

$f (1) = - {(1)}^{2} + 9$

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

One to any power is one.

$f (1) = - 1 \cdot 1 + 9$

Step 2.8.1.2

Multiply

$- 1$ by

$1$ .

$f (1) = - 1 + 9$

Step 2.8.2

Add

$- 1$ and

$9$ .

$f (1) = 8$

Step 2.8.3

The final answer is

$8$ .

$8$

Step 2.9

The

$y$ value at

$x = 1$ is

$8$ .

$y = 8$

Step 2.10

Replace the variable

$x$ with

$2$ in the expression.

$f (2) = - {(2)}^{2} + 9$

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

$2$ to the power of

$2$ .

$f (2) = - 1 \cdot 4 + 9$

Step 2.11.1.2

Multiply

$- 1$ by

$4$ .

$f (2) = - 4 + 9$

Step 2.11.2

Add

$- 4$ and

$9$ .

$f (2) = 5$

Step 2.11.3

The final answer is

$5$ .

$5$

Step 2.12

The

$y$ value at

$x = 2$ is

$5$ .

$y = 5$

Step 2.13

Graph the parabola using its properties and the selected points.

$\begin{array}{c} x & y \\ - 2 & 5 \\ - 1 & 8 \\ 0 & 9 \\ 1 & 8 \\ 2 & 5 \end{array}$

Step 3

Graph the parabola using its properties and the selected points.

Direction: Opens Down

Vertex:

$(0, 9)$

Focus:

$(0, \frac{35}{4})$

Axis of Symmetry:

$x = 0$

Directrix:

$y = \frac{37}{4}$

$\begin{array}{c} x & y \\ - 2 & 5 \\ - 1 & 8 \\ 0 & 9 \\ 1 & 8 \\ 2 & 5 \end{array}$

Step 4