Calculus Examples

$f (x) = - 2 x^{3} + 21 x^{2} - 36 x$ on $0$ , $7$

Step 1

Find the critical points.

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Step 1.1

Find the first 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 $- 2 x^{3} + 21 x^{2} - 36 x$ with respect to $x$ is $\frac{d}{d x} [- 2 x^{3}] + \frac{d}{d x} [21 x^{2}] + \frac{d}{d x} [- 36 x]$ .

$\frac{d}{d x} [- 2 x^{3}] + \frac{d}{d x} [21 x^{2}] + \frac{d}{d x} [- 36 x]$

Step 1.1.1.2

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

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Step 1.1.1.2.1

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

$- 2 \frac{d}{d x} [x^{3}] + \frac{d}{d x} [21 x^{2}] + \frac{d}{d x} [- 36 x]$

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 $n = 3$ .

$- 2 (3 x^{2}) + \frac{d}{d x} [21 x^{2}] + \frac{d}{d x} [- 36 x]$

Step 1.1.1.2.3

Multiply $3$ by $- 2$ .

$- 6 x^{2} + \frac{d}{d x} [21 x^{2}] + \frac{d}{d x} [- 36 x]$

Step 1.1.1.3

Evaluate $\frac{d}{d x} [21 x^{2}]$ .

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Step 1.1.1.3.1

Since $21$ is constant with respect to $x$ , the derivative of $21 x^{2}$ with respect to $x$ is $21 \frac{d}{d x} [x^{2}]$ .

$- 6 x^{2} + 21 \frac{d}{d x} [x^{2}] + \frac{d}{d x} [- 36 x]$

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 $n = 2$ .

$- 6 x^{2} + 21 (2 x) + \frac{d}{d x} [- 36 x]$

Step 1.1.1.3.3

Multiply $2$ by $21$ .

$- 6 x^{2} + 42 x + \frac{d}{d x} [- 36 x]$

Step 1.1.1.4

Evaluate $\frac{d}{d x} [- 36 x]$ .

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Step 1.1.1.4.1

Since $- 36$ is constant with respect to $x$ , the derivative of $- 36 x$ with respect to $x$ is $- 36 \frac{d}{d x} [x]$ .

$- 6 x^{2} + 42 x - 36 \frac{d}{d x} [x]$

Step 1.1.1.4.2

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

$- 6 x^{2} + 42 x - 36 \cdot 1$

Step 1.1.1.4.3

Multiply $- 36$ by $1$ .

$f' (x) = - 6 x^{2} + 42 x - 36$

Step 1.1.2

The first derivative of $f (x)$ with respect to $x$ is $- 6 x^{2} + 42 x - 36$ .

$- 6 x^{2} + 42 x - 36$

Step 1.2

Set the first derivative equal to $0$ then solve the equation $- 6 x^{2} + 42 x - 36 = 0$ .

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Step 1.2.1

Set the first derivative equal to $0$ .

$- 6 x^{2} + 42 x - 36 = 0$

Step 1.2.2

Factor the left side of the equation.

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Step 1.2.2.1

Factor $- 6$ out of $- 6 x^{2} + 42 x - 36$ .

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Step 1.2.2.1.1

Factor $- 6$ out of $- 6 x^{2}$ .

$- 6 x^{2} + 42 x - 36 = 0$

Step 1.2.2.1.2

Factor $- 6$ out of $42 x$ .

$- 6 x^{2} - 6 (- 7 x) - 36 = 0$

Step 1.2.2.1.3

Factor $- 6$ out of $- 36$ .

$- 6 x^{2} - 6 (- 7 x) - 6 \cdot 6 = 0$

Step 1.2.2.1.4

Factor $- 6$ out of $- 6 (x^{2}) - 6 (- 7 x)$ .

$- 6 (x^{2} - 7 x) - 6 \cdot 6 = 0$

Step 1.2.2.1.5

Factor $- 6$ out of $- 6 (x^{2} - 7 x) - 6 (6)$ .

$- 6 (x^{2} - 7 x + 6) = 0$

Step 1.2.2.2

Factor.

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Step 1.2.2.2.1

Factor $x^{2} - 7 x + 6$ using the AC method.

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Step 1.2.2.2.1.1

Consider the form $x^{2} + b x + c$ . Find a pair of integers whose product is $c$ and whose sum is $b$ . In this case, whose product is $6$ and whose sum is $- 7$ .

$- 6, - 1$

Step 1.2.2.2.1.2

Write the factored form using these integers.

$- 6 ((x - 6) (x - 1)) = 0$

Step 1.2.2.2.2

Remove unnecessary parentheses.

$- 6 (x - 6) (x - 1) = 0$

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 $0$ .

$x - 6 = 0$

$x - 1 = 0$

Step 1.2.4

Set $x - 6$ equal to $0$ and solve for $x$ .

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Step 1.2.4.1

Set $x - 6$ equal to $0$ .

$x - 6 = 0$

Step 1.2.4.2

Add $6$ to both sides of the equation.

$x = 6$

Step 1.2.5

Set $x - 1$ equal to $0$ and solve for $x$ .

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Step 1.2.5.1

Set $x - 1$ equal to $0$ .

$x - 1 = 0$

Step 1.2.5.2

Add $1$ to both sides of the equation.

$x = 1$

Step 1.2.6

The final solution is all the values that make $- 6 (x - 6) (x - 1) = 0$ true.

$x = 6, 1$

Step 1.3

Find the values where the derivative is undefined.

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Step 1.3.1

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 1.4

Evaluate $- 2 x^{3} + 21 x^{2} - 36 x$ at each $x$ value where the derivative is $0$ or undefined.

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Step 1.4.1

Evaluate at $x = 6$ .

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Step 1.4.1.1

Substitute $6$ for $x$ .

$- 2 {(6)}^{3} + 21 {(6)}^{2} - 36 \cdot 6$

Step 1.4.1.2

Simplify.

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Step 1.4.1.2.1

Simplify each term.

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Step 1.4.1.2.1.1

Raise $6$ to the power of $3$ .

$- 2 \cdot 216 + 21 {(6)}^{2} - 36 \cdot 6$

Step 1.4.1.2.1.2

Multiply $- 2$ by $216$ .

$- 432 + 21 {(6)}^{2} - 36 \cdot 6$

Step 1.4.1.2.1.3

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

$- 432 + 21 \cdot 36 - 36 \cdot 6$

Step 1.4.1.2.1.4

Multiply $21$ by $36$ .

$- 432 + 756 - 36 \cdot 6$

Step 1.4.1.2.1.5

Multiply $- 36$ by $6$ .

$- 432 + 756 - 216$

Step 1.4.1.2.2

Simplify by adding and subtracting.

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Step 1.4.1.2.2.1

Add $- 432$ and $756$ .

$324 - 216$

Step 1.4.1.2.2.2

Subtract $216$ from $324$ .

$108$

Step 1.4.2

Evaluate at $x = 1$ .

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Step 1.4.2.1

Substitute $1$ for $x$ .

$- 2 {(1)}^{3} + 21 {(1)}^{2} - 36 \cdot 1$

Step 1.4.2.2

Simplify.

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Step 1.4.2.2.1

Simplify each term.

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Step 1.4.2.2.1.1

One to any power is one.

$- 2 \cdot 1 + 21 {(1)}^{2} - 36 \cdot 1$

Step 1.4.2.2.1.2

Multiply $- 2$ by $1$ .

$- 2 + 21 {(1)}^{2} - 36 \cdot 1$

Step 1.4.2.2.1.3

One to any power is one.

$- 2 + 21 \cdot 1 - 36 \cdot 1$

Step 1.4.2.2.1.4

Multiply $21$ by $1$ .

$- 2 + 21 - 36 \cdot 1$

Step 1.4.2.2.1.5

Multiply $- 36$ by $1$ .

$- 2 + 21 - 36$

Step 1.4.2.2.2

Simplify by adding and subtracting.

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Step 1.4.2.2.2.1

Add $- 2$ and $21$ .

$19 - 36$

Step 1.4.2.2.2.2

Subtract $36$ from $19$ .

$- 17$

Step 1.4.3

List all of the points.

$(6, 108), (1, - 17)$

Step 2

Evaluate at the included endpoints.

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Step 2.1

Evaluate at $x = 0$ .

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Step 2.1.1

Substitute $0$ for $x$ .

$- 2 {(0)}^{3} + 21 {(0)}^{2} - 36 \cdot 0$

Step 2.1.2

Simplify.

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Step 2.1.2.1

Simplify each term.

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Step 2.1.2.1.1

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

$- 2 \cdot 0 + 21 {(0)}^{2} - 36 \cdot 0$

Step 2.1.2.1.2

Multiply $- 2$ by $0$ .

$0 + 21 {(0)}^{2} - 36 \cdot 0$

Step 2.1.2.1.3

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

$0 + 21 \cdot 0 - 36 \cdot 0$

Step 2.1.2.1.4

Multiply $21$ by $0$ .

$0 + 0 - 36 \cdot 0$

Step 2.1.2.1.5

Multiply $- 36$ by $0$ .

$0 + 0 + 0$

Step 2.1.2.2

Simplify by adding numbers.

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Step 2.1.2.2.1

Add $0$ and $0$ .

$0 + 0$

Step 2.1.2.2.2

Add $0$ and $0$ .

$0$

Step 2.2

Evaluate at $x = 7$ .

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Step 2.2.1

Substitute $7$ for $x$ .

$- 2 {(7)}^{3} + 21 {(7)}^{2} - 36 \cdot 7$

Step 2.2.2

Simplify.

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Step 2.2.2.1

Simplify each term.

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Step 2.2.2.1.1

Raise $7$ to the power of $3$ .

$- 2 \cdot 343 + 21 {(7)}^{2} - 36 \cdot 7$

Step 2.2.2.1.2

Multiply $- 2$ by $343$ .

$- 686 + 21 {(7)}^{2} - 36 \cdot 7$

Step 2.2.2.1.3

Raise $7$ to the power of $2$ .

$- 686 + 21 \cdot 49 - 36 \cdot 7$

Step 2.2.2.1.4

Multiply $21$ by $49$ .

$- 686 + 1029 - 36 \cdot 7$

Step 2.2.2.1.5

Multiply $- 36$ by $7$ .

$- 686 + 1029 - 252$

Step 2.2.2.2

Simplify by adding and subtracting.

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Step 2.2.2.2.1

Add $- 686$ and $1029$ .

$343 - 252$

Step 2.2.2.2.2

Subtract $252$ from $343$ .

$91$

Step 2.3

List all of the points.

$(0, 0), (7, 91)$

Step 3

Compare the $f (x)$ values found for each value of $x$ in order to determine the absolute maximum and minimum over the given interval. The maximum will occur at the highest $f (x)$ value and the minimum will occur at the lowest $f (x)$ value.

Absolute Maximum: $(6, 108)$

Absolute Minimum: $(1, - 17)$

Step 4