Calculus Examples

$f (x) = 3 x^{4} + 8 x^{3}$

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

By the Sum Rule, the derivative of $3 x^{4} + 8 x^{3}$ with respect to $x$ is $\frac{d}{d x} [3 x^{4}] + \frac{d}{d x} [8 x^{3}]$ .

$\frac{d}{d x} [3 x^{4}] + \frac{d}{d x} [8 x^{3}]$

Step 1.1.1.2

Evaluate $\frac{d}{d x} [3 x^{4}]$ .

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Step 1.1.1.2.1

Since $3$ is constant with respect to $x$ , the derivative of $3 x^{4}$ with respect to $x$ is $3 \frac{d}{d x} [x^{4}]$ .

$3 \frac{d}{d x} [x^{4}] + \frac{d}{d x} [8 x^{3}]$

Step 1.1.1.2.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 4$ .

$3 (4 x^{3}) + \frac{d}{d x} [8 x^{3}]$

Step 1.1.1.2.3

Multiply $4$ by $3$ .

$12 x^{3} + \frac{d}{d x} [8 x^{3}]$

Step 1.1.1.3

Evaluate $\frac{d}{d x} [8 x^{3}]$ .

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Step 1.1.1.3.1

Since $8$ is constant with respect to $x$ , the derivative of $8 x^{3}$ with respect to $x$ is $8 \frac{d}{d x} [x^{3}]$ .

$12 x^{3} + 8 \frac{d}{d x} [x^{3}]$

Step 1.1.1.3.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 3$ .

$12 x^{3} + 8 (3 x^{2})$

Step 1.1.1.3.3

Multiply $3$ by $8$ .

$f' (x) = 12 x^{3} + 24 x^{2}$

Step 1.1.2

Find the second derivative.

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Step 1.1.2.1

By the Sum Rule, the derivative of $12 x^{3} + 24 x^{2}$ with respect to $x$ is $\frac{d}{d x} [12 x^{3}] + \frac{d}{d x} [24 x^{2}]$ .

$\frac{d}{d x} [12 x^{3}] + \frac{d}{d x} [24 x^{2}]$

Step 1.1.2.2

Evaluate $\frac{d}{d x} [12 x^{3}]$ .

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Step 1.1.2.2.1

Since $12$ is constant with respect to $x$ , the derivative of $12 x^{3}$ with respect to $x$ is $12 \frac{d}{d x} [x^{3}]$ .

$12 \frac{d}{d x} [x^{3}] + \frac{d}{d x} [24 x^{2}]$

Step 1.1.2.2.2

Differentiate using the Power Rule which states that $\frac{d}{d x} [x^{n}]$ is $n x^{n - 1}$ where $n = 3$ .

$12 (3 x^{2}) + \frac{d}{d x} [24 x^{2}]$

Step 1.1.2.2.3

Multiply $3$ by $12$ .

$36 x^{2} + \frac{d}{d x} [24 x^{2}]$

Step 1.1.2.3

Evaluate $\frac{d}{d x} [24 x^{2}]$ .

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Step 1.1.2.3.1

Since $24$ is constant with respect to $x$ , the derivative of $24 x^{2}$ with respect to $x$ is $24 \frac{d}{d x} [x^{2}]$ .

$36 x^{2} + 24 \frac{d}{d x} [x^{2}]$

Step 1.1.2.3.2

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

$36 x^{2} + 24 (2 x)$

Step 1.1.2.3.3

Multiply $2$ by $24$ .

$f'' (x) = 36 x^{2} + 48 x$

Step 1.1.3

The second derivative of $f (x)$ with respect to $x$ is $36 x^{2} + 48 x$ .

$36 x^{2} + 48 x$

Step 1.2

Set the second derivative equal to $0$ then solve the equation $36 x^{2} + 48 x = 0$ .

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Step 1.2.1

Set the second derivative equal to $0$ .

$36 x^{2} + 48 x = 0$

Step 1.2.2

Factor $12 x$ out of $36 x^{2} + 48 x$ .

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Step 1.2.2.1

Factor $12 x$ out of $36 x^{2}$ .

$12 x (3 x) + 48 x = 0$

Step 1.2.2.2

Factor $12 x$ out of $48 x$ .

$12 x (3 x) + 12 x (4) = 0$

Step 1.2.2.3

Factor $12 x$ out of $12 x (3 x) + 12 x (4)$ .

$12 x (3 x + 4) = 0$

Step 1.2.3

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

$x = 0$

$3 x + 4 = 0$

Step 1.2.4

Set $x$ equal to $0$ .

$x = 0$

Step 1.2.5

Set $3 x + 4$ equal to $0$ and solve for $x$ .

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Step 1.2.5.1

Set $3 x + 4$ equal to $0$ .

$3 x + 4 = 0$

Step 1.2.5.2

Solve $3 x + 4 = 0$ for $x$ .

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Step 1.2.5.2.1

Subtract $4$ from both sides of the equation.

$3 x = - 4$

Step 1.2.5.2.2

Divide each term in $3 x = - 4$ by $3$ and simplify.

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Step 1.2.5.2.2.1

Divide each term in $3 x = - 4$ by $3$ .

$\frac{3 x}{3} = \frac{- 4}{3}$

Step 1.2.5.2.2.2

Simplify the left side.

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Step 1.2.5.2.2.2.1

Cancel the common factor of $3$ .

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Step 1.2.5.2.2.2.1.1

Cancel the common factor.

$\frac{3 x}{3} = \frac{- 4}{3}$

Step 1.2.5.2.2.2.1.2

Divide $x$ by $1$ .

$x = \frac{- 4}{3}$

Step 1.2.5.2.2.3

Simplify the right side.

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Step 1.2.5.2.2.3.1

Move the negative in front of the fraction.

$x = - \frac{4}{3}$

Step 1.2.6

The final solution is all the values that make $12 x (3 x + 4) = 0$ true.

$x = 0, - \frac{4}{3}$

Step 2

The domain of the expression is all real numbers except where the expression is undefined. In this case, there is no real number that makes the expression undefined.

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.

$(- \infty, - \frac{4}{3}) \cup (- \frac{4}{3}, 0) \cup (0, \infty)$

Step 4

Substitute any number from the interval $(- \infty, - \frac{4}{3})$ into the second derivative and evaluate to determine the concavity.

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Step 4.1

Replace the variable $x$ with $- 4$ in the expression.

$f'' (- 4) = 36 {(- 4)}^{2} + 48 (- 4)$

Step 4.2

Simplify the result.

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Step 4.2.1

Simplify each term.

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Step 4.2.1.1

Raise $- 4$ to the power of $2$ .

$f'' (- 4) = 36 \cdot 16 + 48 (- 4)$

Step 4.2.1.2

Multiply $36$ by $16$ .

$f'' (- 4) = 576 + 48 (- 4)$

Step 4.2.1.3

Multiply $48$ by $- 4$ .

$f'' (- 4) = 576 - 192$

Step 4.2.2

Subtract $192$ from $576$ .

$f'' (- 4) = 384$

Step 4.2.3

The final answer is $384$ .

$384$

Step 4.3

The graph is concave up on the interval $(- \infty, - \frac{4}{3})$ because $f'' (- 4)$ is positive.

Concave up on $(- \infty, - \frac{4}{3})$ since $f'' (x)$ is positive

Step 5

Substitute any number from the interval $(- \frac{4}{3}, 0)$ into the second derivative and evaluate to determine the concavity.

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Step 5.1

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

$f'' (- 1) = 36 {(- 1)}^{2} + 48 (- 1)$

Step 5.2

Simplify the result.

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Step 5.2.1

Simplify each term.

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Step 5.2.1.1

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

$f'' (- 1) = 36 \cdot 1 + 48 (- 1)$

Step 5.2.1.2

Multiply $36$ by $1$ .

$f'' (- 1) = 36 + 48 (- 1)$

Step 5.2.1.3

Multiply $48$ by $- 1$ .

$f'' (- 1) = 36 - 48$

Step 5.2.2

Subtract $48$ from $36$ .

$f'' (- 1) = - 12$

Step 5.2.3

The final answer is $- 12$ .

$- 12$

Step 5.3

The graph is concave down on the interval $(- \frac{4}{3}, 0)$ because $f'' (- 1)$ is negative.

Concave down on $(- \frac{4}{3}, 0)$ since $f'' (x)$ is negative

Step 6

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

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Step 6.1

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

$f'' (2) = 36 {(2)}^{2} + 48 (2)$

Step 6.2

Simplify the result.

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Step 6.2.1

Simplify each term.

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Step 6.2.1.1

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

$f'' (2) = 36 \cdot 4 + 48 (2)$

Step 6.2.1.2

Multiply $36$ by $4$ .

$f'' (2) = 144 + 48 (2)$

Step 6.2.1.3

Multiply $48$ by $2$ .

$f'' (2) = 144 + 96$

Step 6.2.2

Add $144$ and $96$ .

$f'' (2) = 240$

Step 6.2.3

The final answer is $240$ .

$240$

Step 6.3

The graph is concave up on the interval $(0, \infty)$ because $f'' (2)$ is positive.

Concave up on $(0, \infty)$ since $f'' (x)$ is positive

Step 7

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

Concave up on $(- \infty, - \frac{4}{3})$ since $f'' (x)$ is positive

Concave down on $(- \frac{4}{3}, 0)$ since $f'' (x)$ is negative

Concave up on $(0, \infty)$ since $f'' (x)$ is positive

Step 8