Calculus Examples

$f (x) = x^{2} - 4 x$ , $[2, 6)$

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

Differentiate.

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Step 1.1.1.1.1

By the Sum Rule, the derivative of $x^{2} - 4 x$ with respect to $x$ is $\frac{d}{d x} [x^{2}] + \frac{d}{d x} [- 4 x]$ .

$\frac{d}{d x} [x^{2}] + \frac{d}{d x} [- 4 x]$

Step 1.1.1.1.2

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

$2 x + \frac{d}{d x} [- 4 x]$

Step 1.1.1.2

Evaluate $\frac{d}{d x} [- 4 x]$ .

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Step 1.1.1.2.1

Since $- 4$ is constant with respect to $x$ , the derivative of $- 4 x$ with respect to $x$ is $- 4 \frac{d}{d x} [x]$ .

$2 x - 4 \frac{d}{d x} [x]$

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 $n = 1$ .

$2 x - 4 \cdot 1$

Step 1.1.1.2.3

Multiply $- 4$ by $1$ .

$f' (x) = 2 x - 4$

Step 1.1.2

The first derivative of $f (x)$ with respect to $x$ is $2 x - 4$ .

$2 x - 4$

Step 1.2

Set the first derivative equal to $0$ then solve the equation $2 x - 4 = 0$ .

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Step 1.2.1

Set the first derivative equal to $0$ .

$2 x - 4 = 0$

Step 1.2.2

Add $4$ to both sides of the equation.

$2 x = 4$

Step 1.2.3

Divide each term in $2 x = 4$ by $2$ and simplify.

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Step 1.2.3.1

Divide each term in $2 x = 4$ by $2$ .

$\frac{2 x}{2} = \frac{4}{2}$

Step 1.2.3.2

Simplify the left side.

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Step 1.2.3.2.1

Cancel the common factor of $2$ .

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Step 1.2.3.2.1.1

Cancel the common factor.

$\frac{2 x}{2} = \frac{4}{2}$

Step 1.2.3.2.1.2

Divide $x$ by $1$ .

$x = \frac{4}{2}$

Step 1.2.3.3

Simplify the right side.

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Step 1.2.3.3.1

Divide $4$ by $2$ .

$x = 2$

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 $x^{2} - 4 x$ at each $x$ value where the derivative is $0$ or undefined.

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Step 1.4.1

Evaluate at $x = 2$ .

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Step 1.4.1.1

Substitute $2$ for $x$ .

${(2)}^{2} - 4 \cdot 2$

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 $2$ to the power of $2$ .

$4 - 4 \cdot 2$

Step 1.4.1.2.1.2

Multiply $- 4$ by $2$ .

$4 - 8$

Step 1.4.1.2.2

Subtract $8$ from $4$ .

$- 4$

Step 1.4.2

List all of the points.

$(2, - 4)$

Step 2

Since there is no value of $x$ that makes the first derivative equal to $0$ , there are no local extrema.

No Local Extrema

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.

No absolute maximum

Absolute Minimum: $(2, - 4)$

Step 4