Precalculus Examples

$f (x) = x^{2} + 9 x - 16$

Step 1

The minimum of a quadratic function occurs at $x = - \frac{b}{2 a}$ . If $a$ is positive, the minimum value of the function is $f (- \frac{b}{2 a})$ .

$f_{min}$ $x = a x^{2} + b x + c$ occurs at $x = - \frac{b}{2 a}$

Step 2

Find the value of $x = - \frac{b}{2 a}$ .

Tap for more steps...

Step 2.1

Substitute in the values of $a$ and $b$ .

$x = - \frac{9}{2 (1)}$

Step 2.2

Remove parentheses.

$x = - \frac{9}{2 (1)}$

Step 2.3

Multiply $2$ by $1$ .

$x = - \frac{9}{2}$

Step 3

Evaluate $f (- \frac{9}{2})$ .

Tap for more steps...

Step 3.1

Replace the variable $x$ with $- \frac{9}{2}$ in the expression.

$f (- \frac{9}{2}) = {(- \frac{9}{2})}^{2} + 9 (- \frac{9}{2}) - 16$

Step 3.2

Simplify the result.

Tap for more steps...

Step 3.2.1

Simplify each term.

Tap for more steps...

Step 3.2.1.1

Use the power rule ${(a b)}^{n} = a^{n} b^{n}$ to distribute the exponent.

Tap for more steps...

Step 3.2.1.1.1

Apply the product rule to $- \frac{9}{2}$ .

$f (- \frac{9}{2}) = {(- 1)}^{2} {(\frac{9}{2})}^{2} + 9 (- \frac{9}{2}) - 16$

Step 3.2.1.1.2

Apply the product rule to $\frac{9}{2}$ .

$f (- \frac{9}{2}) = {(- 1)}^{2} (\frac{9^{2}}{2^{2}}) + 9 (- \frac{9}{2}) - 16$

Step 3.2.1.2

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

$f (- \frac{9}{2}) = 1 (\frac{9^{2}}{2^{2}}) + 9 (- \frac{9}{2}) - 16$

Step 3.2.1.3

Multiply $\frac{9^{2}}{2^{2}}$ by $1$ .

$f (- \frac{9}{2}) = \frac{9^{2}}{2^{2}} + 9 (- \frac{9}{2}) - 16$

Step 3.2.1.4

Raise $9$ to the power of $2$ .

$f (- \frac{9}{2}) = \frac{81}{2^{2}} + 9 (- \frac{9}{2}) - 16$

Step 3.2.1.5

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

$f (- \frac{9}{2}) = \frac{81}{4} + 9 (- \frac{9}{2}) - 16$

Step 3.2.1.6

Multiply $9 (- \frac{9}{2})$ .

Tap for more steps...

Step 3.2.1.6.1

Multiply $- 1$ by $9$ .

$f (- \frac{9}{2}) = \frac{81}{4} - 9 (\frac{9}{2}) - 16$

Step 3.2.1.6.2

Combine $- 9$ and $\frac{9}{2}$ .

$f (- \frac{9}{2}) = \frac{81}{4} + \frac{- 9 \cdot 9}{2} - 16$

Step 3.2.1.6.3

Multiply $- 9$ by $9$ .

$f (- \frac{9}{2}) = \frac{81}{4} + \frac{- 81}{2} - 16$

Step 3.2.1.7

Move the negative in front of the fraction.

$f (- \frac{9}{2}) = \frac{81}{4} - \frac{81}{2} - 16$

Step 3.2.2

Find the common denominator.

Tap for more steps...

Step 3.2.2.1

Multiply $\frac{81}{2}$ by $\frac{2}{2}$ .

$f (- \frac{9}{2}) = \frac{81}{4} - (\frac{81}{2} \cdot \frac{2}{2}) - 16$

Step 3.2.2.2

Multiply $\frac{81}{2}$ by $\frac{2}{2}$ .

$f (- \frac{9}{2}) = \frac{81}{4} - \frac{81 \cdot 2}{2 \cdot 2} - 16$

Step 3.2.2.3

Write $- 16$ as a fraction with denominator $1$ .

$f (- \frac{9}{2}) = \frac{81}{4} - \frac{81 \cdot 2}{2 \cdot 2} + \frac{- 16}{1}$

Step 3.2.2.4

Multiply $\frac{- 16}{1}$ by $\frac{4}{4}$ .

$f (- \frac{9}{2}) = \frac{81}{4} - \frac{81 \cdot 2}{2 \cdot 2} + \frac{- 16}{1} \cdot \frac{4}{4}$

Step 3.2.2.5

Multiply $\frac{- 16}{1}$ by $\frac{4}{4}$ .

$f (- \frac{9}{2}) = \frac{81}{4} - \frac{81 \cdot 2}{2 \cdot 2} + \frac{- 16 \cdot 4}{4}$

Step 3.2.2.6

Multiply $2$ by $2$ .

$f (- \frac{9}{2}) = \frac{81}{4} - \frac{81 \cdot 2}{4} + \frac{- 16 \cdot 4}{4}$

Step 3.2.3

Combine the numerators over the common denominator.

$f (- \frac{9}{2}) = \frac{81 - 81 \cdot 2 - 16 \cdot 4}{4}$

Step 3.2.4

Simplify each term.

Tap for more steps...

Step 3.2.4.1

Multiply $- 81$ by $2$ .

$f (- \frac{9}{2}) = \frac{81 - 162 - 16 \cdot 4}{4}$

Step 3.2.4.2

Multiply $- 16$ by $4$ .

$f (- \frac{9}{2}) = \frac{81 - 162 - 64}{4}$

Step 3.2.5

Simplify the expression.

Tap for more steps...

Step 3.2.5.1

Subtract $162$ from $81$ .

$f (- \frac{9}{2}) = \frac{- 81 - 64}{4}$

Step 3.2.5.2

Subtract $64$ from $- 81$ .

$f (- \frac{9}{2}) = \frac{- 145}{4}$

Step 3.2.5.3

Move the negative in front of the fraction.

$f (- \frac{9}{2}) = - \frac{145}{4}$

Step 3.2.6

The final answer is $- \frac{145}{4}$ .

$- \frac{145}{4}$

Step 4

Use the $x$ and $y$ values to find where the minimum occurs.

$(- \frac{9}{2}, - \frac{145}{4})$

Step 5