Calculus Examples

$f (x) = 5 x^{3} - 15 x$

Step 1

Find the first derivative.

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Step 1.1

Find the first derivative.

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Step 1.1.1

By the Sum Rule, the derivative of $5 x^{3} - 15 x$ with respect to $x$ is $\frac{d}{d x} [5 x^{3}] + \frac{d}{d x} [- 15 x]$ .

$\frac{d}{d x} [5 x^{3}] + \frac{d}{d x} [- 15 x]$

Step 1.1.2

Evaluate $\frac{d}{d x} [5 x^{3}]$ .

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Step 1.1.2.1

Since $5$ is constant with respect to $x$ , the derivative of $5 x^{3}$ with respect to $x$ is $5 \frac{d}{d x} [x^{3}]$ .

$5 \frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 15 x]$

Step 1.1.2.2

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

$5 (3 x^{2}) + \frac{d}{d x} [- 15 x]$

Step 1.1.2.3

Multiply $3$ by $5$ .

$15 x^{2} + \frac{d}{d x} [- 15 x]$

Step 1.1.3

Evaluate $\frac{d}{d x} [- 15 x]$ .

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Step 1.1.3.1

Since $- 15$ is constant with respect to $x$ , the derivative of $- 15 x$ with respect to $x$ is $- 15 \frac{d}{d x} [x]$ .

$15 x^{2} - 15 \frac{d}{d x} [x]$

Step 1.1.3.2

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

$15 x^{2} - 15 \cdot 1$

Step 1.1.3.3

Multiply $- 15$ by $1$ .

$f' (x) = 15 x^{2} - 15$

Step 1.2

The first derivative of $f (x)$ with respect to $x$ is $15 x^{2} - 15$ .

$15 x^{2} - 15$

Step 2

Set the first derivative equal to $0$ then solve the equation $15 x^{2} - 15 = 0$ .

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Step 2.1

Set the first derivative equal to $0$ .

$15 x^{2} - 15 = 0$

Step 2.2

Add $15$ to both sides of the equation.

$15 x^{2} = 15$

Step 2.3

Divide each term in $15 x^{2} = 15$ by $15$ and simplify.

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Step 2.3.1

Divide each term in $15 x^{2} = 15$ by $15$ .

$\frac{15 x^{2}}{15} = \frac{15}{15}$

Step 2.3.2

Simplify the left side.

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Step 2.3.2.1

Cancel the common factor of $15$ .

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Step 2.3.2.1.1

Cancel the common factor.

$\frac{15 x^{2}}{15} = \frac{15}{15}$

Step 2.3.2.1.2

Divide $x^{2}$ by $1$ .

$x^{2} = \frac{15}{15}$

Step 2.3.3

Simplify the right side.

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Step 2.3.3.1

Divide $15$ by $15$ .

$x^{2} = 1$

Step 2.4

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

$x = \pm \sqrt{1}$

Step 2.5

Any root of $1$ is $1$ .

$x = \pm 1$

Step 2.6

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

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Step 2.6.1

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

$x = 1$

Step 2.6.2

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

$x = - 1$

Step 2.6.3

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

$x = 1, - 1$

Step 3

Find the values where the derivative is undefined.

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Step 3.1

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.

Step 4

Evaluate $5 x^{3} - 15 x$ at each $x$ value where the derivative is $0$ or undefined.

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Step 4.1

Evaluate at $x = 1$ .

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Step 4.1.1

Substitute $1$ for $x$ .

$5 {(1)}^{3} - 15 \cdot 1$

Step 4.1.2

Simplify.

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Step 4.1.2.1

Simplify each term.

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Step 4.1.2.1.1

One to any power is one.

$5 \cdot 1 - 15 \cdot 1$

Step 4.1.2.1.2

Multiply $5$ by $1$ .

$5 - 15 \cdot 1$

Step 4.1.2.1.3

Multiply $- 15$ by $1$ .

$5 - 15$

Step 4.1.2.2

Subtract $15$ from $5$ .

$- 10$

Step 4.2

Evaluate at $x = - 1$ .

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Step 4.2.1

Substitute $- 1$ for $x$ .

$5 {(- 1)}^{3} - 15 \cdot - 1$

Step 4.2.2

Simplify.

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Step 4.2.2.1

Simplify each term.

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Step 4.2.2.1.1

Raise $- 1$ to the power of $3$ .

$5 \cdot - 1 - 15 \cdot - 1$

Step 4.2.2.1.2

Multiply $5$ by $- 1$ .

$- 5 - 15 \cdot - 1$

Step 4.2.2.1.3

Multiply $- 15$ by $- 1$ .

$- 5 + 15$

Step 4.2.2.2

Add $- 5$ and $15$ .

$10$

Step 4.3

List all of the points.

$(1, - 10), (- 1, 10)$

Step 5