Precalculus Examples

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

Step 1

Write $f (x) = x^{2} - 8 x$ as an equation.

$y = x^{2} - 8 x$

Step 2

Since $x$ is on the right side of the equation, switch the sides so it is on the left side of the equation.

$x^{2} - 8 x = y$

Step 3

Complete the square.

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Step 3.1

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

$a = 1$

$b = - 8$

$c = 0 = y$

Step 3.2

Consider the vertex form of a parabola.

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

Step 3.3

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

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Step 3.3.1

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

$d = \frac{- 8}{2 \cdot 1}$

Step 3.3.2

Cancel the common factor of $- 8$ and $2$ .

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Step 3.3.2.1

Factor $2$ out of $- 8$ .

$d = \frac{2 \cdot - 4}{2 \cdot 1}$

Step 3.3.2.2

Cancel the common factors.

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Step 3.3.2.2.1

Factor $2$ out of $2 \cdot 1$ .

$d = \frac{2 \cdot - 4}{2 (1)}$

Step 3.3.2.2.2

Cancel the common factor.

$d = \frac{2 \cdot - 4}{2 \cdot 1}$

Step 3.3.2.2.3

Rewrite the expression.

$d = \frac{- 4}{1}$

Step 3.3.2.2.4

Divide $- 4$ by $1$ .

$d = - 4$

Step 3.4

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

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Step 3.4.1

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

$e = 0 - \frac{{(- 8)}^{2}}{4 \cdot 1}$

Step 3.4.2

Simplify the right side.

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Step 3.4.2.1

Simplify each term.

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Step 3.4.2.1.1

Raise $- 8$ to the power of $2$ .

$e = 0 - \frac{64}{4 \cdot 1}$

Step 3.4.2.1.2

Multiply $4$ by $1$ .

$e = 0 - \frac{64}{4}$

Step 3.4.2.1.3

Divide $64$ by $4$ .

$e = 0 - 1 \cdot 16$

Step 3.4.2.1.4

Multiply $- 1$ by $16$ .

$e = 0 - 16$

Step 3.4.2.2

Subtract $16$ from $0$ .

$e = - 16$

Step 3.5

Substitute the values of $a$ , $d$ , and $e$ into the vertex form ${(x - 4)}^{2} - 16$ .

${(x - 4)}^{2} - 16 = y$

Step 4

Add $16$ to both sides of the equation.

${(x - 4)}^{2} = y + 16$