Calculus Examples

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

Step 1

Find the $x$ values where the second derivative is equal to $0$ .

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Step 1.1

Find the second derivative.

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Step 1.1.1

Find the first derivative.

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Step 1.1.1.1

Differentiate using the Quotient Rule which states that $\frac{d}{d x} [\frac{f (x)}{g (x)}]$ is $\frac{g (x) \frac{d}{d x} [f (x)] - f (x) \frac{d}{d x} [g (x)]}{{g (x)}^{2}}$ where $f (x) = x$ and $g (x) = x^{2} + 9$ .

$f' (x) = \frac{(x^{2} + 9) \frac{d}{d x} (x) - x \frac{d}{d x} (x^{2} + 9)}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.2

Differentiate.

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Step 1.1.1.2.1

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 1$ .

$f' (x) = \frac{(x^{2} + 9) \cdot 1 - x \frac{d}{d x} (x^{2} + 9)}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.2.2

Multiply $x^{2} + 9$ by $1$ .

$f' (x) = \frac{x^{2} + 9 - x \frac{d}{d x} (x^{2} + 9)}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.2.3

By the Sum Rule, the derivative of $x^{2} + 9$ with respect to $x$ is $\frac{d}{d x} [x^{2}] + \frac{d}{d x} [9]$ .

$f' (x) = \frac{x^{2} + 9 - x (\frac{d}{d x} (x^{2}) + \frac{d}{d x} (9))}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.2.4

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 2$ .

$f' (x) = \frac{x^{2} + 9 - x (2 x + \frac{d}{d x} (9))}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.2.5

Since $9$ is constant with respect to $x$ , the derivative of $9$ with respect to $x$ is $0$ .

$f' (x) = \frac{x^{2} + 9 - x (2 x + 0)}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.2.6

Simplify the expression.

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Step 1.1.1.2.6.1

Add $2 x$ and $0$ .

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

Step 1.1.1.2.6.2

Multiply $2$ by $- 1$ .

$f' (x) = \frac{x^{2} + 9 - 2 x \cdot x}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.3

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

$f' (x) = \frac{x^{2} + 9 - 2 (x \cdot x)}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.4

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

$f' (x) = \frac{x^{2} + 9 - 2 (x \cdot x)}{{(x^{2} + 9)}^{2}}$

Step 1.1.1.5

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

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

Step 1.1.1.6

Add $1$ and $1$ .

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

Step 1.1.1.7

Subtract $2 x^{2}$ from $x^{2}$ .

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

Step 1.1.2

Find the second derivative.

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Step 1.1.2.1

$f'' (x) = \frac{{(x^{2} + 9)}^{2} \frac{d}{d x} (- x^{2} + 9) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{({(x^{2} + 9)}^{2})}^{2}}$

Step 1.1.2.2

Differentiate.

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Step 1.1.2.2.1

Multiply the exponents in ${({(x^{2} + 9)}^{2})}^{2}$ .

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Step 1.1.2.2.1.1

Apply the power rule and multiply exponents, ${(a^{m})}^{n} = a^{m n}$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} \frac{d}{d x} (- x^{2} + 9) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{2 \cdot 2}}$

Step 1.1.2.2.1.2

Multiply $2$ by $2$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} \frac{d}{d x} (- x^{2} + 9) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.2.2

By the Sum Rule, the derivative of $- x^{2} + 9$ with respect to $x$ is $\frac{d}{d x} [- x^{2}] + \frac{d}{d x} [9]$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (\frac{d}{d x} (- x^{2}) + \frac{d}{d x} (9)) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.2.3

Since $- 1$ is constant with respect to $x$ , the derivative of $- x^{2}$ with respect to $x$ is $- \frac{d}{d x} [x^{2}]$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- \frac{d}{d x} x^{2} + \frac{d}{d x} (9)) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.2.4

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 2$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- (2 x) + \frac{d}{d x} (9)) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.2.5

Multiply $2$ by $- 1$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x + \frac{d}{d x} (9)) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.2.6

Since $9$ is constant with respect to $x$ , the derivative of $9$ with respect to $x$ is $0$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x + 0) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.2.7

Add $- 2 x$ and $0$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - (- x^{2} + 9) \frac{d}{d x} {(x^{2} + 9)}^{2}}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.3

Differentiate using the chain rule, which states that $\frac{d}{d x} [f (g (x))]$ is $f' (g (x)) g' (x)$ where $f (x) = x^{2}$ and $g (x) = x^{2} + 9$ .

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Step 1.1.2.3.1

To apply the Chain Rule, set $u$ as $x^{2} + 9$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - (- x^{2} + 9) (\frac{d}{d u} (u^{2}) \frac{d}{d x} (x^{2} + 9))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.3.2

Differentiate using the Power Rule which states that $\frac{d}{d u} [u^{n}]$ is $n u^{n - 1}$ where $n = 2$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - (- x^{2} + 9) (2 u \frac{d}{d x} (x^{2} + 9))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.3.3

Replace all occurrences of $u$ with $x^{2} + 9$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - (- x^{2} + 9) (2 (x^{2} + 9) \frac{d}{d x} (x^{2} + 9))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.4

Differentiate.

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Step 1.1.2.4.1

Multiply $2$ by $- 1$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - 2 (- x^{2} + 9) ((x^{2} + 9) \frac{d}{d x} (x^{2} + 9))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.4.2

By the Sum Rule, the derivative of $x^{2} + 9$ with respect to $x$ is $\frac{d}{d x} [x^{2}] + \frac{d}{d x} [9]$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - 2 (- x^{2} + 9) ((x^{2} + 9) (\frac{d}{d x} (x^{2}) + \frac{d}{d x} (9)))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.4.3

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 2$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - 2 (- x^{2} + 9) ((x^{2} + 9) (2 x + \frac{d}{d x} (9)))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.4.4

Since $9$ is constant with respect to $x$ , the derivative of $9$ with respect to $x$ is $0$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - 2 (- x^{2} + 9) ((x^{2} + 9) (2 x + 0))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.4.5

Simplify the expression.

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Step 1.1.2.4.5.1

Add $2 x$ and $0$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - 2 (- x^{2} + 9) ((x^{2} + 9) (2 x))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.4.5.2

Move $2$ to the left of $x^{2} + 9$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - 2 (- x^{2} + 9) (2 \cdot ((x^{2} + 9) x))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.4.5.3

Multiply $2$ by $- 2$ .

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) - 4 (- x^{2} + 9) ((x^{2} + 9) x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5

Simplify.

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Step 1.1.2.5.1

Apply the distributive property.

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) + (- 4 (- x^{2}) - 4 \cdot 9) ((x^{2} + 9) x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.2

Apply the distributive property.

$f'' (x) = \frac{{(x^{2} + 9)}^{2} (- 2 x) + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3

Simplify the numerator.

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Step 1.1.2.5.3.1

Simplify each term.

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Step 1.1.2.5.3.1.1

Rewrite using the commutative property of multiplication.

$f'' (x) = \frac{- 2 {(x^{2} + 9)}^{2} x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.2

Rewrite ${(x^{2} + 9)}^{2}$ as $(x^{2} + 9) (x^{2} + 9)$ .

$f'' (x) = \frac{- 2 ((x^{2} + 9) (x^{2} + 9)) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.3

Expand $(x^{2} + 9) (x^{2} + 9)$ using the FOIL Method.

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Step 1.1.2.5.3.1.3.1

Apply the distributive property.

$f'' (x) = \frac{- 2 (x^{2} (x^{2} + 9) + 9 (x^{2} + 9)) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.3.2

Apply the distributive property.

$f'' (x) = \frac{- 2 (x^{2} x^{2} + x^{2} \cdot 9 + 9 (x^{2} + 9)) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.3.3

Apply the distributive property.

$f'' (x) = \frac{- 2 (x^{2} x^{2} + x^{2} \cdot 9 + 9 x^{2} + 9 \cdot 9) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.4

Simplify and combine like terms.

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Step 1.1.2.5.3.1.4.1

Simplify each term.

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Step 1.1.2.5.3.1.4.1.1

Multiply $x^{2}$ by $x^{2}$ by adding the exponents.

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Step 1.1.2.5.3.1.4.1.1.1

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

$f'' (x) = \frac{- 2 (x^{2 + 2} + x^{2} \cdot 9 + 9 x^{2} + 9 \cdot 9) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.4.1.1.2

Add $2$ and $2$ .

$f'' (x) = \frac{- 2 (x^{4} + x^{2} \cdot 9 + 9 x^{2} + 9 \cdot 9) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.4.1.2

Move $9$ to the left of $x^{2}$ .

$f'' (x) = \frac{- 2 (x^{4} + 9 \cdot x^{2} + 9 x^{2} + 9 \cdot 9) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.4.1.3

Multiply $9$ by $9$ .

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

Step 1.1.2.5.3.1.4.2

Add $9 x^{2}$ and $9 x^{2}$ .

$f'' (x) = \frac{- 2 (x^{4} + 18 x^{2} + 81) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.5

Apply the distributive property.

$f'' (x) = \frac{(- 2 x^{4} - 2 (18 x^{2}) - 2 \cdot 81) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.6

Simplify.

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Step 1.1.2.5.3.1.6.1

Multiply $18$ by $- 2$ .

$f'' (x) = \frac{(- 2 x^{4} - 36 x^{2} - 2 \cdot 81) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.6.2

Multiply $- 2$ by $81$ .

$f'' (x) = \frac{(- 2 x^{4} - 36 x^{2} - 162) x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.7

Apply the distributive property.

$f'' (x) = \frac{- 2 x^{4} x - 36 x^{2} x - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8

Simplify.

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Step 1.1.2.5.3.1.8.1

Multiply $x^{4}$ by $x$ by adding the exponents.

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Step 1.1.2.5.3.1.8.1.1

Move $x$ .

$f'' (x) = \frac{- 2 (x \cdot x^{4}) - 36 x^{2} x - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8.1.2

Multiply $x$ by $x^{4}$ .

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Step 1.1.2.5.3.1.8.1.2.1

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

$f'' (x) = \frac{- 2 (x \cdot x^{4}) - 36 x^{2} x - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8.1.2.2

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

$f'' (x) = \frac{- 2 x^{1 + 4} - 36 x^{2} x - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8.1.3

Add $1$ and $4$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{2} x - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8.2

Multiply $x^{2}$ by $x$ by adding the exponents.

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Step 1.1.2.5.3.1.8.2.1

Move $x$ .

$f'' (x) = \frac{- 2 x^{5} - 36 (x \cdot x^{2}) - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8.2.2

Multiply $x$ by $x^{2}$ .

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Step 1.1.2.5.3.1.8.2.2.1

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

$f'' (x) = \frac{- 2 x^{5} - 36 (x \cdot x^{2}) - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8.2.2.2

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

$f'' (x) = \frac{- 2 x^{5} - 36 x^{1 + 2} - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.8.2.3

Add $1$ and $2$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + (- 4 (- x^{2}) - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.9

Simplify each term.

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Step 1.1.2.5.3.1.9.1

Multiply $- 1$ by $- 4$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + (4 x^{2} - 4 \cdot 9) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.9.2

Multiply $- 4$ by $9$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + (4 x^{2} - 36) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.10

Multiply $x^{2}$ by $x$ by adding the exponents.

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Step 1.1.2.5.3.1.10.1

Multiply $x^{2}$ by $x$ .

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Step 1.1.2.5.3.1.10.1.1

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

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + (4 x^{2} - 36) (x^{2} x + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.10.1.2

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

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + (4 x^{2} - 36) (x^{2 + 1} + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.10.2

Add $2$ and $1$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + (4 x^{2} - 36) (x^{3} + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.11

Expand $(4 x^{2} - 36) (x^{3} + 9 x)$ using the FOIL Method.

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Step 1.1.2.5.3.1.11.1

Apply the distributive property.

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{2} (x^{3} + 9 x) - 36 (x^{3} + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.11.2

Apply the distributive property.

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{2} x^{3} + 4 x^{2} (9 x) - 36 (x^{3} + 9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.11.3

Apply the distributive property.

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{2} x^{3} + 4 x^{2} (9 x) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12

Simplify and combine like terms.

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Step 1.1.2.5.3.1.12.1

Simplify each term.

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Step 1.1.2.5.3.1.12.1.1

Multiply $x^{2}$ by $x^{3}$ by adding the exponents.

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Step 1.1.2.5.3.1.12.1.1.1

Move $x^{3}$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 (x^{3} x^{2}) + 4 x^{2} (9 x) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.1.2

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

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{3 + 2} + 4 x^{2} (9 x) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.1.3

Add $3$ and $2$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 4 x^{2} (9 x) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.2

Rewrite using the commutative property of multiplication.

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 4 \cdot (9 x^{2} x) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.3

Multiply $x^{2}$ by $x$ by adding the exponents.

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Step 1.1.2.5.3.1.12.1.3.1

Move $x$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 4 \cdot (9 (x \cdot x^{2})) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.3.2

Multiply $x$ by $x^{2}$ .

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Step 1.1.2.5.3.1.12.1.3.2.1

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

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 4 \cdot (9 (x \cdot x^{2})) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.3.2.2

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

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 4 \cdot (9 x^{1 + 2}) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.3.3

Add $1$ and $2$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 4 \cdot (9 x^{3}) - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.4

Multiply $4$ by $9$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 36 x^{3} - 36 x^{3} - 36 (9 x)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.1.5

Multiply $9$ by $- 36$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 36 x^{3} - 36 x^{3} - 324 x}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.2

Subtract $36 x^{3}$ from $36 x^{3}$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} + 0 - 324 x}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.1.12.3

Add $4 x^{5}$ and $0$ .

$f'' (x) = \frac{- 2 x^{5} - 36 x^{3} - 162 x + 4 x^{5} - 324 x}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.2

Add $- 2 x^{5}$ and $4 x^{5}$ .

$f'' (x) = \frac{2 x^{5} - 36 x^{3} - 162 x - 324 x}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.3.3

Subtract $324 x$ from $- 162 x$ .

$f'' (x) = \frac{2 x^{5} - 36 x^{3} - 486 x}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4

Simplify the numerator.

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Step 1.1.2.5.4.1

Factor $2 x$ out of $2 x^{5} - 36 x^{3} - 486 x$ .

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Step 1.1.2.5.4.1.1

Factor $2 x$ out of $2 x^{5}$ .

$f'' (x) = \frac{2 x (x^{4}) - 36 x^{3} - 486 x}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.1.2

Factor $2 x$ out of $- 36 x^{3}$ .

$f'' (x) = \frac{2 x (x^{4}) + 2 x (- 18 x^{2}) - 486 x}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.1.3

Factor $2 x$ out of $- 486 x$ .

$f'' (x) = \frac{2 x (x^{4}) + 2 x (- 18 x^{2}) + 2 x (- 243)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.1.4

Factor $2 x$ out of $2 x (x^{4}) + 2 x (- 18 x^{2})$ .

$f'' (x) = \frac{2 x (x^{4} - 18 x^{2}) + 2 x (- 243)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.1.5

Factor $2 x$ out of $2 x (x^{4} - 18 x^{2}) + 2 x (- 243)$ .

$f'' (x) = \frac{2 x (x^{4} - 18 x^{2} - 243)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.2

Rewrite $x^{4}$ as ${(x^{2})}^{2}$ .

$f'' (x) = \frac{2 x ({(x^{2})}^{2} - 18 x^{2} - 243)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.3

Let $u = x^{2}$ . Substitute $u$ for all occurrences of $x^{2}$ .

$f'' (x) = \frac{2 x (u^{2} - 18 u - 243)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.4

Factor $u^{2} - 18 u - 243$ using the AC method.

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Step 1.1.2.5.4.4.1

Consider the form $x^{2} + b x + c$ . Find a pair of integers whose product is $c$ and whose sum is $b$ . In this case, whose product is $- 243$ and whose sum is $- 18$ .

$- 27, 9$

Step 1.1.2.5.4.4.2

Write the factored form using these integers.

$f'' (x) = \frac{2 x ((u - 27) (u + 9))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.4.5

Replace all occurrences of $u$ with $x^{2}$ .

$f'' (x) = \frac{2 x (x^{2} - 27) (x^{2} + 9)}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.5

Cancel the common factor of $x^{2} + 9$ and ${(x^{2} + 9)}^{4}$ .

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Step 1.1.2.5.5.1

Factor $x^{2} + 9$ out of $2 x (x^{2} - 27) (x^{2} + 9)$ .

$f'' (x) = \frac{(x^{2} + 9) (2 x (x^{2} - 27))}{{(x^{2} + 9)}^{4}}$

Step 1.1.2.5.5.2

Cancel the common factors.

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Step 1.1.2.5.5.2.1

Factor $x^{2} + 9$ out of ${(x^{2} + 9)}^{4}$ .

$f'' (x) = \frac{(x^{2} + 9) (2 x (x^{2} - 27))}{(x^{2} + 9) {(x^{2} + 9)}^{3}}$

Step 1.1.2.5.5.2.2

Cancel the common factor.

$f'' (x) = \frac{(x^{2} + 9) (2 x (x^{2} - 27))}{(x^{2} + 9) {(x^{2} + 9)}^{3}}$

Step 1.1.2.5.5.2.3

Rewrite the expression.

$f'' (x) = \frac{2 x (x^{2} - 27)}{{(x^{2} + 9)}^{3}}$

Step 1.1.3

The second derivative of $f (x)$ with respect to $x$ is $\frac{2 x (x^{2} - 27)}{{(x^{2} + 9)}^{3}}$ .

$\frac{2 x (x^{2} - 27)}{{(x^{2} + 9)}^{3}}$

Step 1.2

Set the second derivative equal to $0$ then solve the equation $\frac{2 x (x^{2} - 27)}{{(x^{2} + 9)}^{3}} = 0$ .

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Step 1.2.1

Set the second derivative equal to $0$ .

$\frac{2 x (x^{2} - 27)}{{(x^{2} + 9)}^{3}} = 0$

Step 1.2.2

Set the numerator equal to zero.

$2 x (x^{2} - 27) = 0$

Step 1.2.3

Solve the equation for $x$ .

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Step 1.2.3.1

If any individual factor on the left side of the equation is equal to $0$ , the entire expression will be equal to $0$ .

$x = 0$

$x^{2} - 27 = 0$

Step 1.2.3.2

Set $x$ equal to $0$ .

$x = 0$

Step 1.2.3.3

Set $x^{2} - 27$ equal to $0$ and solve for $x$ .

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Step 1.2.3.3.1

Set $x^{2} - 27$ equal to $0$ .

$x^{2} - 27 = 0$

Step 1.2.3.3.2

Solve $x^{2} - 27 = 0$ for $x$ .

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Step 1.2.3.3.2.1

Add $27$ to both sides of the equation.

$x^{2} = 27$

Step 1.2.3.3.2.2

Take the specified root of both sides of the equation to eliminate the exponent on the left side.

$x = \pm \sqrt{27}$

Step 1.2.3.3.2.3

Simplify $\pm \sqrt{27}$ .

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Step 1.2.3.3.2.3.1

Rewrite $27$ as $3^{2} \cdot 3$ .

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Step 1.2.3.3.2.3.1.1

Factor $9$ out of $27$ .

$x = \pm \sqrt{9 (3)}$

Step 1.2.3.3.2.3.1.2

Rewrite $9$ as $3^{2}$ .

$x = \pm \sqrt{3^{2} \cdot 3}$

Step 1.2.3.3.2.3.2

Pull terms out from under the radical.

$x = \pm 3 \sqrt{3}$

Step 1.2.3.3.2.4

The complete solution is the result of both the positive and negative portions of the solution.

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Step 1.2.3.3.2.4.1

First, use the positive value of the $\pm$ to find the first solution.

$x = 3 \sqrt{3}$

Step 1.2.3.3.2.4.2

Next, use the negative value of the $\pm$ to find the second solution.

$x = - 3 \sqrt{3}$

Step 1.2.3.3.2.4.3

The complete solution is the result of both the positive and negative portions of the solution.

$x = 3 \sqrt{3}, - 3 \sqrt{3}$

Step 1.2.3.4

The final solution is all the values that make $2 x (x^{2} - 27) = 0$ true.

$x = 0, 3 \sqrt{3}, - 3 \sqrt{3}$

Step 2

Find the domain of $f (x) = \frac{x}{x^{2} + 9}$ .

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Step 2.1

Set the denominator in $\frac{x}{x^{2} + 9}$ equal to $0$ to find where the expression is undefined.

$x^{2} + 9 = 0$

Step 2.2

Solve for $x$ .

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Step 2.2.1

Subtract $9$ from both sides of the equation.

$x^{2} = - 9$

Step 2.2.2

Take the specified root of both sides of the equation to eliminate the exponent on the left side.

$x = \pm \sqrt{- 9}$

Step 2.2.3

Simplify $\pm \sqrt{- 9}$ .

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Step 2.2.3.1

Rewrite $- 9$ as $- 1 (9)$ .

$x = \pm \sqrt{- 1 (9)}$

Step 2.2.3.2

Rewrite $\sqrt{- 1 (9)}$ as $\sqrt{- 1} \cdot \sqrt{9}$ .

$x = \pm \sqrt{- 1} \cdot \sqrt{9}$

Step 2.2.3.3

Rewrite $\sqrt{- 1}$ as $i$ .

$x = \pm i \cdot \sqrt{9}$

Step 2.2.3.4

Rewrite $9$ as $3^{2}$ .

$x = \pm i \cdot \sqrt{3^{2}}$

Step 2.2.3.5

Pull terms out from under the radical, assuming positive real numbers.

$x = \pm i \cdot 3$

Step 2.2.3.6

Move $3$ to the left of $i$ .

$x = \pm 3 i$

Step 2.2.4

The complete solution is the result of both the positive and negative portions of the solution.

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Step 2.2.4.1

First, use the positive value of the $\pm$ to find the first solution.

$x = 3 i$

Step 2.2.4.2

Next, use the negative value of the $\pm$ to find the second solution.

$x = - 3 i$

Step 2.2.4.3

The complete solution is the result of both the positive and negative portions of the solution.

$x = 3 i, - 3 i$

Step 2.3

The domain is all real numbers.

Interval Notation:

$(- \infty, \infty)$

Set-Builder Notation:

${x | x \in ℝ}$

Interval Notation:

$(- \infty, \infty)$

Set-Builder Notation:

${x | x \in ℝ}$

Step 3

Create intervals around the $x$ -values where the second derivative is zero or undefined.

$(- \infty, - 3 \sqrt{3}) \cup (- 3 \sqrt{3}, 0) \cup (0, 3 \sqrt{3}) \cup (3 \sqrt{3}, \infty)$

Step 4

Substitute any number from the interval $(- \infty, - 3 \sqrt{3})$ into the second derivative and evaluate to determine the concavity.

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Step 4.1

Replace the variable $x$ with $- 8$ in the expression.

$f'' (- 8) = \frac{2 (- 8) ({(- 8)}^{2} - 27)}{{({(- 8)}^{2} + 9)}^{3}}$

Step 4.2

Simplify the result.

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Step 4.2.1

Multiply $2$ by $- 8$ .

$f'' (- 8) = \frac{- 16 ({(- 8)}^{2} - 27)}{{({(- 8)}^{2} + 9)}^{3}}$

Step 4.2.2

Simplify the denominator.

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Step 4.2.2.1

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

$f'' (- 8) = \frac{- 16 ({(- 8)}^{2} - 27)}{{(64 + 9)}^{3}}$

Step 4.2.2.2

Add $64$ and $9$ .

$f'' (- 8) = \frac{- 16 ({(- 8)}^{2} - 27)}{73^{3}}$

Step 4.2.2.3

Raise $73$ to the power of $3$ .

$f'' (- 8) = \frac{- 16 ({(- 8)}^{2} - 27)}{389017}$

Step 4.2.3

Simplify the numerator.

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Step 4.2.3.1

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

$f'' (- 8) = \frac{- 16 (64 - 27)}{389017}$

Step 4.2.3.2

Subtract $27$ from $64$ .

$f'' (- 8) = \frac{- 16 \cdot 37}{389017}$

Step 4.2.4

Simplify the expression.

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Step 4.2.4.1

Multiply $- 16$ by $37$ .

$f'' (- 8) = \frac{- 592}{389017}$

Step 4.2.4.2

Move the negative in front of the fraction.

$f'' (- 8) = - \frac{592}{389017}$

Step 4.2.5

The final answer is $- \frac{592}{389017}$ .

$- \frac{592}{389017}$

Step 4.3

The graph is concave down on the interval $(- \infty, - 3 \sqrt{3})$ because $f'' (- 8)$ is negative.

Concave down on $(- \infty, - 3 \sqrt{3})$ since $f'' (x)$ is negative

Step 5

Substitute any number from the interval $(- 3 \sqrt{3}, 0)$ into the second derivative and evaluate to determine the concavity.

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Step 5.1

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

$f'' (- 3) = \frac{2 (- 3) ({(- 3)}^{2} - 27)}{{({(- 3)}^{2} + 9)}^{3}}$

Step 5.2

Simplify the result.

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Step 5.2.1

Multiply $2$ by $- 3$ .

$f'' (- 3) = \frac{- 6 ({(- 3)}^{2} - 27)}{{({(- 3)}^{2} + 9)}^{3}}$

Step 5.2.2

Simplify the denominator.

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Step 5.2.2.1

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

$f'' (- 3) = \frac{- 6 ({(- 3)}^{2} - 27)}{{(9 + 9)}^{3}}$

Step 5.2.2.2

Add $9$ and $9$ .

$f'' (- 3) = \frac{- 6 ({(- 3)}^{2} - 27)}{18^{3}}$

Step 5.2.2.3

Raise $18$ to the power of $3$ .

$f'' (- 3) = \frac{- 6 ({(- 3)}^{2} - 27)}{5832}$

Step 5.2.3

Simplify the numerator.

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Step 5.2.3.1

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

$f'' (- 3) = \frac{- 6 (9 - 27)}{5832}$

Step 5.2.3.2

Subtract $27$ from $9$ .

$f'' (- 3) = \frac{- 6 \cdot - 18}{5832}$

Step 5.2.4

Reduce the expression by cancelling the common factors.

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Step 5.2.4.1

Multiply $- 6$ by $- 18$ .

$f'' (- 3) = \frac{108}{5832}$

Step 5.2.4.2

Cancel the common factor of $108$ and $5832$ .

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Step 5.2.4.2.1

Factor $108$ out of $108$ .

$f'' (- 3) = \frac{108 (1)}{5832}$

Step 5.2.4.2.2

Cancel the common factors.

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Step 5.2.4.2.2.1

Factor $108$ out of $5832$ .

$f'' (- 3) = \frac{108 \cdot 1}{108 \cdot 54}$

Step 5.2.4.2.2.2

Cancel the common factor.

$f'' (- 3) = \frac{108 \cdot 1}{108 \cdot 54}$

Step 5.2.4.2.2.3

Rewrite the expression.

$f'' (- 3) = \frac{1}{54}$

Step 5.2.5

The final answer is $\frac{1}{54}$ .

$\frac{1}{54}$

Step 5.3

The graph is concave up on the interval $(- 3 \sqrt{3}, 0)$ because $f'' (- 3)$ is positive.

Concave up on $(- 3 \sqrt{3}, 0)$ since $f'' (x)$ is positive

Step 6

Substitute any number from the interval $(0, 3 \sqrt{3})$ into the second derivative and evaluate to determine the concavity.

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Step 6.1

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

$f'' (3) = \frac{2 (3) ({(3)}^{2} - 27)}{{({(3)}^{2} + 9)}^{3}}$

Step 6.2

Simplify the result.

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Step 6.2.1

Multiply $2$ by $3$ .

$f'' (3) = \frac{6 (3^{2} - 27)}{{(3^{2} + 9)}^{3}}$

Step 6.2.2

Simplify the denominator.

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Step 6.2.2.1

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

$f'' (3) = \frac{6 (3^{2} - 27)}{{(9 + 9)}^{3}}$

Step 6.2.2.2

Add $9$ and $9$ .

$f'' (3) = \frac{6 (3^{2} - 27)}{18^{3}}$

Step 6.2.2.3

Raise $18$ to the power of $3$ .

$f'' (3) = \frac{6 (3^{2} - 27)}{5832}$

Step 6.2.3

Simplify the numerator.

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Step 6.2.3.1

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

$f'' (3) = \frac{6 (9 - 27)}{5832}$

Step 6.2.3.2

Subtract $27$ from $9$ .

$f'' (3) = \frac{6 \cdot - 18}{5832}$

Step 6.2.4

Reduce the expression by cancelling the common factors.

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Step 6.2.4.1

Multiply $6$ by $- 18$ .

$f'' (3) = \frac{- 108}{5832}$

Step 6.2.4.2

Cancel the common factor of $- 108$ and $5832$ .

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Step 6.2.4.2.1

Factor $108$ out of $- 108$ .

$f'' (3) = \frac{108 (- 1)}{5832}$

Step 6.2.4.2.2

Cancel the common factors.

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Step 6.2.4.2.2.1

Factor $108$ out of $5832$ .

$f'' (3) = \frac{108 \cdot - 1}{108 \cdot 54}$

Step 6.2.4.2.2.2

Cancel the common factor.

$f'' (3) = \frac{108 \cdot - 1}{108 \cdot 54}$

Step 6.2.4.2.2.3

Rewrite the expression.

$f'' (3) = \frac{- 1}{54}$

Step 6.2.4.3

Move the negative in front of the fraction.

$f'' (3) = - \frac{1}{54}$

Step 6.2.5

The final answer is $- \frac{1}{54}$ .

$- \frac{1}{54}$

Step 6.3

The graph is concave down on the interval $(0, 3 \sqrt{3})$ because $f'' (3)$ is negative.

Concave down on $(0, 3 \sqrt{3})$ since $f'' (x)$ is negative

Step 7

Substitute any number from the interval $(3 \sqrt{3}, \infty)$ into the second derivative and evaluate to determine the concavity.

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Step 7.1

Replace the variable $x$ with $8$ in the expression.

$f'' (8) = \frac{2 (8) ({(8)}^{2} - 27)}{{({(8)}^{2} + 9)}^{3}}$

Step 7.2

Simplify the result.

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Step 7.2.1

Multiply $2$ by $8$ .

$f'' (8) = \frac{16 (8^{2} - 27)}{{(8^{2} + 9)}^{3}}$

Step 7.2.2

Simplify the denominator.

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Step 7.2.2.1

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

$f'' (8) = \frac{16 (8^{2} - 27)}{{(64 + 9)}^{3}}$

Step 7.2.2.2

Add $64$ and $9$ .

$f'' (8) = \frac{16 (8^{2} - 27)}{73^{3}}$

Step 7.2.2.3

Raise $73$ to the power of $3$ .

$f'' (8) = \frac{16 (8^{2} - 27)}{389017}$

Step 7.2.3

Simplify the numerator.

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Step 7.2.3.1

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

$f'' (8) = \frac{16 (64 - 27)}{389017}$

Step 7.2.3.2

Subtract $27$ from $64$ .

$f'' (8) = \frac{16 \cdot 37}{389017}$

Step 7.2.4

Multiply $16$ by $37$ .

$f'' (8) = \frac{592}{389017}$

Step 7.2.5

The final answer is $\frac{592}{389017}$ .

$\frac{592}{389017}$

Step 7.3

The graph is concave up on the interval $(3 \sqrt{3}, \infty)$ because $f'' (8)$ is positive.

Concave up on $(3 \sqrt{3}, \infty)$ since $f'' (x)$ is positive

Step 8

The graph is concave down when the second derivative is negative and concave up when the second derivative is positive.

Concave down on $(- \infty, - 3 \sqrt{3})$ since $f'' (x)$ is negative

Concave up on $(- 3 \sqrt{3}, 0)$ since $f'' (x)$ is positive

Concave down on $(0, 3 \sqrt{3})$ since $f'' (x)$ is negative

Concave up on $(3 \sqrt{3}, \infty)$ since $f'' (x)$ is positive

Step 9