Calculus Examples

$f (x) = x^{3} - 75 x + 3$

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

Differentiate.

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Step 1.1.1.1

By the Sum Rule, the derivative of $x^{3} - 75 x + 3$ with respect to $x$ is $\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 75 x] + \frac{d}{d x} [3]$ .

$\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 75 x] + \frac{d}{d x} [3]$

Step 1.1.1.2

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

$3 x^{2} + \frac{d}{d x} [- 75 x] + \frac{d}{d x} [3]$

Step 1.1.2

Evaluate $\frac{d}{d x} [- 75 x]$ .

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Step 1.1.2.1

Since $- 75$ is constant with respect to $x$ , the derivative of $- 75 x$ with respect to $x$ is $- 75 \frac{d}{d x} [x]$ .

$3 x^{2} - 75 \frac{d}{d x} [x] + \frac{d}{d x} [3]$

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 $n = 1$ .

$3 x^{2} - 75 \cdot 1 + \frac{d}{d x} [3]$

Step 1.1.2.3

Multiply $- 75$ by $1$ .

$3 x^{2} - 75 + \frac{d}{d x} [3]$

Step 1.1.3

Differentiate using the Constant Rule.

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Step 1.1.3.1

Since $3$ is constant with respect to $x$ , the derivative of $3$ with respect to $x$ is $0$ .

$3 x^{2} - 75 + 0$

Step 1.1.3.2

Add $3 x^{2} - 75$ and $0$ .

$f' (x) = 3 x^{2} - 75$

Step 1.2

The first derivative of $f (x)$ with respect to $x$ is $3 x^{2} - 75$ .

$3 x^{2} - 75$

Step 2

Set the first derivative equal to $0$ then solve the equation $3 x^{2} - 75 = 0$ .

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Step 2.1

Set the first derivative equal to $0$ .

$3 x^{2} - 75 = 0$

Step 2.2

Add $75$ to both sides of the equation.

$3 x^{2} = 75$

Step 2.3

Divide each term in $3 x^{2} = 75$ by $3$ and simplify.

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Step 2.3.1

Divide each term in $3 x^{2} = 75$ by $3$ .

$\frac{3 x^{2}}{3} = \frac{75}{3}$

Step 2.3.2

Simplify the left side.

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Step 2.3.2.1

Cancel the common factor of $3$ .

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Step 2.3.2.1.1

Cancel the common factor.

$\frac{3 x^{2}}{3} = \frac{75}{3}$

Step 2.3.2.1.2

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

$x^{2} = \frac{75}{3}$

Step 2.3.3

Simplify the right side.

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Step 2.3.3.1

Divide $75$ by $3$ .

$x^{2} = 25$

Step 2.4

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

$x = \pm \sqrt{25}$

Step 2.5

Simplify $\pm \sqrt{25}$ .

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Step 2.5.1

Rewrite $25$ as $5^{2}$ .

$x = \pm \sqrt{5^{2}}$

Step 2.5.2

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

$x = \pm 5$

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.

$x = 5$

Step 2.6.2

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

$x = - 5$

Step 2.6.3

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

$x = 5, - 5$

Step 3

The values which make the derivative equal to $0$ are $5, - 5$ .

$5, - 5$

Step 4

Split $(- \infty, \infty)$ into separate intervals around the $x$ values that make the derivative $0$ or undefined.

$(- \infty, - 5) \cup (- 5, 5) \cup (5, \infty)$

Step 5

Substitute a value from the interval $(- \infty, - 5)$ into the derivative to determine if the function is increasing or decreasing.

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Step 5.1

Replace the variable $x$ with $- 6$ in the expression.

$f' (- 6) = 3 {(- 6)}^{2} - 75$

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 $- 6$ to the power of $2$ .

$f' (- 6) = 3 \cdot 36 - 75$

Step 5.2.1.2

Multiply $3$ by $36$ .

$f' (- 6) = 108 - 75$

Step 5.2.2

Subtract $75$ from $108$ .

$f' (- 6) = 33$

Step 5.2.3

The final answer is $33$ .

$33$

Step 5.3

At $x = - 6$ the derivative is $33$ . Since this is positive, the function is increasing on $(- \infty, - 5)$ .

Increasing on $(- \infty, - 5)$ since $f' (x) > 0$

Step 6

Substitute a value from the interval $(- 5, 5)$ into the derivative to determine if the function is increasing or decreasing.

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Step 6.1

Replace the variable $x$ with $0$ in the expression.

$f' (0) = 3 {(0)}^{2} - 75$

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

Raising $0$ to any positive power yields $0$ .

$f' (0) = 3 \cdot 0 - 75$

Step 6.2.1.2

Multiply $3$ by $0$ .

$f' (0) = 0 - 75$

Step 6.2.2

Subtract $75$ from $0$ .

$f' (0) = - 75$

Step 6.2.3

The final answer is $- 75$ .

$- 75$

Step 6.3

At $x = 0$ the derivative is $- 75$ . Since this is negative, the function is decreasing on $(- 5, 5)$ .

Decreasing on $(- 5, 5)$ since $f' (x) < 0$

Step 7

Substitute a value from the interval $(5, \infty)$ into the derivative to determine if the function is increasing or decreasing.

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Step 7.1

Replace the variable $x$ with $6$ in the expression.

$f' (6) = 3 {(6)}^{2} - 75$

Step 7.2

Simplify the result.

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Step 7.2.1

Simplify each term.

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Step 7.2.1.1

Raise $6$ to the power of $2$ .

$f' (6) = 3 \cdot 36 - 75$

Step 7.2.1.2

Multiply $3$ by $36$ .

$f' (6) = 108 - 75$

Step 7.2.2

Subtract $75$ from $108$ .

$f' (6) = 33$

Step 7.2.3

The final answer is $33$ .

$33$

Step 7.3

At $x = 6$ the derivative is $33$ . Since this is positive, the function is increasing on $(5, \infty)$ .

Increasing on $(5, \infty)$ since $f' (x) > 0$

Step 8

List the intervals on which the function is increasing and decreasing.

Increasing on: $(- \infty, - 5), (5, \infty)$

Decreasing on: $(- 5, 5)$

Step 9