Calculus Examples

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

Step 1

Find the first derivative of the function.

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Step 1.1

Differentiate.

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Step 1.1.1

By the Sum Rule, the derivative of $x^{3} - 9 x^{2} + 27 x$ with respect to $x$ is $\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 9 x^{2}] + \frac{d}{d x} [27 x]$ .

$\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 9 x^{2}] + \frac{d}{d x} [27 x]$

Step 1.1.2

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

$3 x^{2} + \frac{d}{d x} [- 9 x^{2}] + \frac{d}{d x} [27 x]$

Step 1.2

Evaluate $\frac{d}{d x} [- 9 x^{2}]$ .

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Step 1.2.1

Since $- 9$ is constant with respect to $x$ , the derivative of $- 9 x^{2}$ with respect to $x$ is $- 9 \frac{d}{d x} [x^{2}]$ .

$3 x^{2} - 9 \frac{d}{d x} [x^{2}] + \frac{d}{d x} [27 x]$

Step 1.2.2

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

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

Step 1.2.3

Multiply $2$ by $- 9$ .

$3 x^{2} - 18 x + \frac{d}{d x} [27 x]$

Step 1.3

Evaluate $\frac{d}{d x} [27 x]$ .

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Step 1.3.1

Since $27$ is constant with respect to $x$ , the derivative of $27 x$ with respect to $x$ is $27 \frac{d}{d x} [x]$ .

$3 x^{2} - 18 x + 27 \frac{d}{d x} [x]$

Step 1.3.2

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

$3 x^{2} - 18 x + 27 \cdot 1$

Step 1.3.3

Multiply $27$ by $1$ .

$3 x^{2} - 18 x + 27$

Step 2

Find the second derivative of the function.

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Step 2.1

By the Sum Rule, the derivative of $3 x^{2} - 18 x + 27$ with respect to $x$ is $\frac{d}{d x} [3 x^{2}] + \frac{d}{d x} [- 18 x] + \frac{d}{d x} [27]$ .

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

Step 2.2

Evaluate $\frac{d}{d x} [3 x^{2}]$ .

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Step 2.2.1

Since $3$ is constant with respect to $x$ , the derivative of $3 x^{2}$ with respect to $x$ is $3 \frac{d}{d x} [x^{2}]$ .

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

Step 2.2.2

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

$f'' (x) = 3 (2 x) + \frac{d}{d x} (- 18 x) + \frac{d}{d x} (27)$

Step 2.2.3

Multiply $2$ by $3$ .

$f'' (x) = 6 x + \frac{d}{d x} (- 18 x) + \frac{d}{d x} (27)$

Step 2.3

Evaluate $\frac{d}{d x} [- 18 x]$ .

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Step 2.3.1

Since $- 18$ is constant with respect to $x$ , the derivative of $- 18 x$ with respect to $x$ is $- 18 \frac{d}{d x} [x]$ .

$f'' (x) = 6 x - 18 \frac{d}{d x} x + \frac{d}{d x} (27)$

Step 2.3.2

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

$f'' (x) = 6 x - 18 \cdot 1 + \frac{d}{d x} (27)$

Step 2.3.3

Multiply $- 18$ by $1$ .

$f'' (x) = 6 x - 18 + \frac{d}{d x} (27)$

Step 2.4

Differentiate using the Constant Rule.

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Step 2.4.1

Since $27$ is constant with respect to $x$ , the derivative of $27$ with respect to $x$ is $0$ .

$f'' (x) = 6 x - 18 + 0$

Step 2.4.2

Add $6 x - 18$ and $0$ .

$f'' (x) = 6 x - 18$

Step 3

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

$3 x^{2} - 18 x + 27 = 0$

Step 4

Find the first derivative.

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Step 4.1

Find the first derivative.

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Step 4.1.1

Differentiate.

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Step 4.1.1.1

By the Sum Rule, the derivative of $x^{3} - 9 x^{2} + 27 x$ with respect to $x$ is $\frac{d}{d x} [x^{3}] + \frac{d}{d x} [- 9 x^{2}] + \frac{d}{d x} [27 x]$ .

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

Step 4.1.1.2

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

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

Step 4.1.2

Evaluate $\frac{d}{d x} [- 9 x^{2}]$ .

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Step 4.1.2.1

Since $- 9$ is constant with respect to $x$ , the derivative of $- 9 x^{2}$ with respect to $x$ is $- 9 \frac{d}{d x} [x^{2}]$ .

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

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 $n = 2$ .

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

Step 4.1.2.3

Multiply $2$ by $- 9$ .

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

Step 4.1.3

Evaluate $\frac{d}{d x} [27 x]$ .

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Step 4.1.3.1

Since $27$ is constant with respect to $x$ , the derivative of $27 x$ with respect to $x$ is $27 \frac{d}{d x} [x]$ .

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

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 $n = 1$ .

$f' (x) = 3 x^{2} - 18 x + 27 \cdot 1$

Step 4.1.3.3

Multiply $27$ by $1$ .

$f' (x) = 3 x^{2} - 18 x + 27$

Step 4.2

The first derivative of $f (x)$ with respect to $x$ is $3 x^{2} - 18 x + 27$ .

$3 x^{2} - 18 x + 27$

Step 5

Set the first derivative equal to $0$ then solve the equation $3 x^{2} - 18 x + 27 = 0$ .

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Step 5.1

Set the first derivative equal to $0$ .

$3 x^{2} - 18 x + 27 = 0$

Step 5.2

Factor the left side of the equation.

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Step 5.2.1

Factor $3$ out of $3 x^{2} - 18 x + 27$ .

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Step 5.2.1.1

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

$3 (x^{2}) - 18 x + 27 = 0$

Step 5.2.1.2

Factor $3$ out of $- 18 x$ .

$3 (x^{2}) + 3 (- 6 x) + 27 = 0$

Step 5.2.1.3

Factor $3$ out of $27$ .

$3 x^{2} + 3 (- 6 x) + 3 \cdot 9 = 0$

Step 5.2.1.4

Factor $3$ out of $3 x^{2} + 3 (- 6 x)$ .

$3 (x^{2} - 6 x) + 3 \cdot 9 = 0$

Step 5.2.1.5

Factor $3$ out of $3 (x^{2} - 6 x) + 3 \cdot 9$ .

$3 (x^{2} - 6 x + 9) = 0$

Step 5.2.2

Factor using the perfect square rule.

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Step 5.2.2.1

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

$3 (x^{2} - 6 x + 3^{2}) = 0$

Step 5.2.2.2

Check that the middle term is two times the product of the numbers being squared in the first term and third term.

$6 x = 2 \cdot x \cdot 3$

Step 5.2.2.3

Rewrite the polynomial.

$3 (x^{2} - 2 \cdot x \cdot 3 + 3^{2}) = 0$

Step 5.2.2.4

Factor using the perfect square trinomial rule $a^{2} - 2 a b + b^{2} = {(a - b)}^{2}$ , where $a = x$ and $b = 3$ .

$3 {(x - 3)}^{2} = 0$

Step 5.3

Divide each term in $3 {(x - 3)}^{2} = 0$ by $3$ and simplify.

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Step 5.3.1

Divide each term in $3 {(x - 3)}^{2} = 0$ by $3$ .

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

Step 5.3.2

Simplify the left side.

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Step 5.3.2.1

Cancel the common factor of $3$ .

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Step 5.3.2.1.1

Cancel the common factor.

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

Step 5.3.2.1.2

Divide ${(x - 3)}^{2}$ by $1$ .

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

Step 5.3.3

Simplify the right side.

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Step 5.3.3.1

Divide $0$ by $3$ .

${(x - 3)}^{2} = 0$

Step 5.4

Set the $x - 3$ equal to $0$ .

$x - 3 = 0$

Step 5.5

Add $3$ to both sides of the equation.

$x = 3$

Step 6

Find the values where the derivative is undefined.

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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.

$x = 3$

Step 8

Evaluate the second derivative at $x = 3$ . If the second derivative is positive, then this is a local minimum. If it is negative, then this is a local maximum.

$6 (3) - 18$

Step 9

Evaluate the second derivative.

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Step 9.1

Multiply $6$ by $3$ .

$18 - 18$

Step 9.2

Subtract $18$ from $18$ .

$0$

Step 10

Since there is at least one point with $0$ or undefined second derivative, apply the first derivative test.

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Step 10.1

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

$(- \infty, 3) \cup (3, \infty)$

Step 10.2

Substitute any number, such as $0$ , from the interval $(- \infty, 3)$ in the first derivative $3 x^{2} - 18 x + 27$ to check if the result is negative or positive.

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Step 10.2.1

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

$f' (0) = 3 {(0)}^{2} - 18 \cdot 0 + 27$

Step 10.2.2

Simplify the result.

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Step 10.2.2.1

Simplify each term.

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Step 10.2.2.1.1

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

$f' (0) = 3 \cdot 0 - 18 \cdot 0 + 27$

Step 10.2.2.1.2

Multiply $3$ by $0$ .

$f' (0) = 0 - 18 \cdot 0 + 27$

Step 10.2.2.1.3

Multiply $- 18$ by $0$ .

$f' (0) = 0 + 0 + 27$

Step 10.2.2.2

Simplify by adding numbers.

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Step 10.2.2.2.1

Add $0$ and $0$ .

$f' (0) = 0 + 27$

Step 10.2.2.2.2

Add $0$ and $27$ .

$f' (0) = 27$

Step 10.2.2.3

The final answer is $27$ .

$27$

Step 10.3

Substitute any number, such as $6$ , from the interval $(3, \infty)$ in the first derivative $3 x^{2} - 18 x + 27$ to check if the result is negative or positive.

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Step 10.3.1

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

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

Step 10.3.2

Simplify the result.

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Step 10.3.2.1

Simplify each term.

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Step 10.3.2.1.1

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

$f' (6) = 3 \cdot 36 - 18 \cdot 6 + 27$

Step 10.3.2.1.2

Multiply $3$ by $36$ .

$f' (6) = 108 - 18 \cdot 6 + 27$

Step 10.3.2.1.3

Multiply $- 18$ by $6$ .

$f' (6) = 108 - 108 + 27$

Step 10.3.2.2

Simplify by adding and subtracting.

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Step 10.3.2.2.1

Subtract $108$ from $108$ .

$f' (6) = 0 + 27$

Step 10.3.2.2.2

Add $0$ and $27$ .

$f' (6) = 27$

Step 10.3.2.3

The final answer is $27$ .

$27$

Step 10.4

Since the first derivative did not change signs around $x = 3$ , this is not a local maximum or minimum.

Not a local maximum or minimum

Step 10.5

No local maxima or minima found for $f (x) = x^{3} - 9 x^{2} + 27 x$ .

No local maxima or minima

Step 11