Calculus Examples

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

Step 1

Find the first derivative of the function.

Tap for more steps...

Step 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 1.2

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

Tap for more steps...

Step 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 1.2.2

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 1.2.3

Multiply $3$ by $5$ .

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

Step 1.3

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

Tap for more steps...

Step 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 1.3.2

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 1.3.3

Multiply $- 15$ by $1$ .

$15 x^{2} - 15$

Step 2

Find the second derivative of the function.

Tap for more steps...

Step 2.1

By the Sum Rule, the derivative of $15 x^{2} - 15$ with respect to $x$ is $\frac{d}{d x} [15 x^{2}] + \frac{d}{d x} [- 15]$ .

$f'' (x) = \frac{d}{d x} (15 x^{2}) + \frac{d}{d x} (- 15)$

Step 2.2

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

Tap for more steps...

Step 2.2.1

Since $15$ is constant with respect to $x$ , the derivative of $15 x^{2}$ with respect to $x$ is $15 \frac{d}{d x} [x^{2}]$ .

$f'' (x) = 15 \frac{d}{d x} (x^{2}) + \frac{d}{d x} (- 15)$

Step 2.2.2

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

$f'' (x) = 15 (2 x) + \frac{d}{d x} (- 15)$

Step 2.2.3

Multiply $2$ by $15$ .

$f'' (x) = 30 x + \frac{d}{d x} (- 15)$

Step 2.3

Differentiate using the Constant Rule.

Tap for more steps...

Step 2.3.1

Since $- 15$ is constant with respect to $x$ , the derivative of $- 15$ with respect to $x$ is $0$ .

$f'' (x) = 30 x + 0$

Step 2.3.2

Add $30 x$ and $0$ .

$f'' (x) = 30 x$

Step 3

To find the local maximum and minimum values of the function, set the derivative equal to $0$ and solve.

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

Step 4

Find the first derivative.

Tap for more steps...

Step 4.1

Find the first derivative.

Tap for more steps...

Step 4.1.1

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 4.1.2

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

Tap for more steps...

Step 4.1.2.1

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 4.1.2.2

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 4.1.2.3

Multiply $3$ by $5$ .

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

Step 4.1.3

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

Tap for more steps...

Step 4.1.3.1

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 4.1.3.2

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 4.1.3.3

Multiply $- 15$ by $1$ .

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

Step 4.2

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

$15 x^{2} - 15$

Step 5

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

Tap for more steps...

Step 5.1

Set the first derivative equal to $0$ .

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

Step 5.2

Add $15$ to both sides of the equation.

$15 x^{2} = 15$

Step 5.3

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

Tap for more steps...

Step 5.3.1

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

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

Step 5.3.2

Simplify the left side.

Tap for more steps...

Step 5.3.2.1

Cancel the common factor of $15$ .

Tap for more steps...

Step 5.3.2.1.1

Cancel the common factor.

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

Step 5.3.2.1.2

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

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

Step 5.3.3

Simplify the right side.

Tap for more steps...

Step 5.3.3.1

Divide $15$ by $15$ .

$x^{2} = 1$

Step 5.4

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

$x = \pm \sqrt{1}$

Step 5.5

Any root of $1$ is $1$ .

$x = \pm 1$

Step 5.6

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

Tap for more steps...

Step 5.6.1

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

$x = 1$

Step 5.6.2

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

$x = - 1$

Step 5.6.3

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

$x = 1, - 1$

Step 6

Find the values where the derivative is undefined.

Tap for more steps...

Step 6.1

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 7

Critical points to evaluate.

$x = 1, - 1$

Step 8

Evaluate the second derivative at $x = 1$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$30 (1)$

Step 9

Multiply $30$ by $1$ .

$30$

Step 10

$x = 1$ is a local minimum because the value of the second derivative is positive. This is referred to as the second derivative test.

$x = 1$ is a local minimum

Step 11

Find the y-value when $x = 1$ .

Tap for more steps...

Step 11.1

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

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

Step 11.2

Simplify the result.

Tap for more steps...

Step 11.2.1

Simplify each term.

Tap for more steps...

Step 11.2.1.1

One to any power is one.

$f (1) = 5 \cdot 1 - 15 \cdot 1$

Step 11.2.1.2

Multiply $5$ by $1$ .

$f (1) = 5 - 15 \cdot 1$

Step 11.2.1.3

Multiply $- 15$ by $1$ .

$f (1) = 5 - 15$

Step 11.2.2

Subtract $15$ from $5$ .

$f (1) = - 10$

Step 11.2.3

The final answer is $- 10$ .

$y = - 10$

Step 12

Evaluate the second derivative at $x = - 1$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$30 (- 1)$

Step 13

Multiply $30$ by $- 1$ .

$- 30$

Step 14

$x = - 1$ is a local maximum because the value of the second derivative is negative. This is referred to as the second derivative test.

$x = - 1$ is a local maximum

Step 15

Find the y-value when $x = - 1$ .

Tap for more steps...

Step 15.1

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

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

Step 15.2

Simplify the result.

Tap for more steps...

Step 15.2.1

Simplify each term.

Tap for more steps...

Step 15.2.1.1

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

$f (- 1) = 5 \cdot - 1 - 15 \cdot - 1$

Step 15.2.1.2

Multiply $5$ by $- 1$ .

$f (- 1) = - 5 - 15 \cdot - 1$

Step 15.2.1.3

Multiply $- 15$ by $- 1$ .

$f (- 1) = - 5 + 15$

Step 15.2.2

Add $- 5$ and $15$ .

$f (- 1) = 10$

Step 15.2.3

The final answer is $10$ .

$y = 10$

Step 16

These are the local extrema for $f (x) = 5 x^{3} - 15 x$ .

$(1, - 10)$ is a local minima

$(- 1, 10)$ is a local maxima

Step 17