Calculus Examples

$f (x) = - 5 x^{2} + 4 x - 1$

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

By the Sum Rule, the derivative of $- 5 x^{2} + 4 x - 1$ with respect to $x$ is $\frac{d}{d x} [- 5 x^{2}] + \frac{d}{d x} [4 x] + \frac{d}{d x} [- 1]$ .

$\frac{d}{d x} [- 5 x^{2}] + \frac{d}{d x} [4 x] + \frac{d}{d x} [- 1]$

Step 1.1.2

Evaluate $\frac{d}{d x} [- 5 x^{2}]$ .

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Step 1.1.2.1

Since $- 5$ is constant with respect to $x$ , the derivative of $- 5 x^{2}$ with respect to $x$ is $- 5 \frac{d}{d x} [x^{2}]$ .

$- 5 \frac{d}{d x} [x^{2}] + \frac{d}{d x} [4 x] + \frac{d}{d x} [- 1]$

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 $n = 2$ .

$- 5 (2 x) + \frac{d}{d x} [4 x] + \frac{d}{d x} [- 1]$

Step 1.1.2.3

Multiply $2$ by $- 5$ .

$- 10 x + \frac{d}{d x} [4 x] + \frac{d}{d x} [- 1]$

Step 1.1.3

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

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Step 1.1.3.1

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

$- 10 x + 4 \frac{d}{d x} [x] + \frac{d}{d x} [- 1]$

Step 1.1.3.2

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

$- 10 x + 4 \cdot 1 + \frac{d}{d x} [- 1]$

Step 1.1.3.3

Multiply $4$ by $1$ .

$- 10 x + 4 + \frac{d}{d x} [- 1]$

Step 1.1.4

Differentiate using the Constant Rule.

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Step 1.1.4.1

Since $- 1$ is constant with respect to $x$ , the derivative of $- 1$ with respect to $x$ is $0$ .

$- 10 x + 4 + 0$

Step 1.1.4.2

Add $- 10 x + 4$ and $0$ .

$f' (x) = - 10 x + 4$

Step 1.2

The first derivative of $f (x)$ with respect to $x$ is $- 10 x + 4$ .

$- 10 x + 4$

Step 2

Set the first derivative equal to $0$ then solve the equation $- 10 x + 4 = 0$ .

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Step 2.1

Set the first derivative equal to $0$ .

$- 10 x + 4 = 0$

Step 2.2

Subtract $4$ from both sides of the equation.

$- 10 x = - 4$

Step 2.3

Divide each term in $- 10 x = - 4$ by $- 10$ and simplify.

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Step 2.3.1

Divide each term in $- 10 x = - 4$ by $- 10$ .

$\frac{- 10 x}{- 10} = \frac{- 4}{- 10}$

Step 2.3.2

Simplify the left side.

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Step 2.3.2.1

Cancel the common factor of $- 10$ .

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Step 2.3.2.1.1

Cancel the common factor.

$\frac{- 10 x}{- 10} = \frac{- 4}{- 10}$

Step 2.3.2.1.2

Divide $x$ by $1$ .

$x = \frac{- 4}{- 10}$

Step 2.3.3

Simplify the right side.

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Step 2.3.3.1

Cancel the common factor of $- 4$ and $- 10$ .

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Step 2.3.3.1.1

Factor $- 2$ out of $- 4$ .

$x = \frac{- 2 (2)}{- 10}$

Step 2.3.3.1.2

Cancel the common factors.

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Step 2.3.3.1.2.1

Factor $- 2$ out of $- 10$ .

$x = \frac{- 2 \cdot 2}{- 2 \cdot 5}$

Step 2.3.3.1.2.2

Cancel the common factor.

$x = \frac{- 2 \cdot 2}{- 2 \cdot 5}$

Step 2.3.3.1.2.3

Rewrite the expression.

$x = \frac{2}{5}$

Step 3

Find the values where the derivative is undefined.

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Step 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 4

Evaluate $- 5 x^{2} + 4 x - 1$ at each $x$ value where the derivative is $0$ or undefined.

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Step 4.1

Evaluate at $x = \frac{2}{5}$ .

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Step 4.1.1

Substitute $\frac{2}{5}$ for $x$ .

$- 5 {(\frac{2}{5})}^{2} + 4 (\frac{2}{5}) - 1$

Step 4.1.2

Simplify.

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Step 4.1.2.1

Simplify each term.

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Step 4.1.2.1.1

Apply the product rule to $\frac{2}{5}$ .

$- 5 \frac{2^{2}}{5^{2}} + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.2

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

$- 5 \frac{4}{5^{2}} + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.3

Raise $5$ to the power of $2$ .

$- 5 (\frac{4}{25}) + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.4

Cancel the common factor of $5$ .

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Step 4.1.2.1.4.1

Factor $5$ out of $- 5$ .

$5 (- 1) \frac{4}{25} + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.4.2

Factor $5$ out of $25$ .

$5 \cdot - 1 \frac{4}{5 \cdot 5} + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.4.3

Cancel the common factor.

$5 \cdot - 1 \frac{4}{5 \cdot 5} + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.4.4

Rewrite the expression.

$- 1 (\frac{4}{5}) + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.5

Rewrite $- 1 (\frac{4}{5})$ as $- (\frac{4}{5})$ .

$- (\frac{4}{5}) + 4 (\frac{2}{5}) - 1$

Step 4.1.2.1.6

Multiply $4 (\frac{2}{5})$ .

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Step 4.1.2.1.6.1

Combine $4$ and $\frac{2}{5}$ .

$- \frac{4}{5} + \frac{4 \cdot 2}{5} - 1$

Step 4.1.2.1.6.2

Multiply $4$ by $2$ .

$- \frac{4}{5} + \frac{8}{5} - 1$

Step 4.1.2.2

Combine fractions.

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Step 4.1.2.2.1

Combine the numerators over the common denominator.

$- 1 + \frac{- 4 + 8}{5}$

Step 4.1.2.2.2

Add $- 4$ and $8$ .

$- 1 + \frac{4}{5}$

Step 4.1.2.3

To write $- 1$ as a fraction with a common denominator, multiply by $\frac{5}{5}$ .

$- 1 \cdot \frac{5}{5} + \frac{4}{5}$

Step 4.1.2.4

Combine $- 1$ and $\frac{5}{5}$ .

$\frac{- 1 \cdot 5}{5} + \frac{4}{5}$

Step 4.1.2.5

Combine the numerators over the common denominator.

$\frac{- 1 \cdot 5 + 4}{5}$

Step 4.1.2.6

Simplify the numerator.

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Step 4.1.2.6.1

Multiply $- 1$ by $5$ .

$\frac{- 5 + 4}{5}$

Step 4.1.2.6.2

Add $- 5$ and $4$ .

$\frac{- 1}{5}$

Step 4.1.2.7

Move the negative in front of the fraction.

$- \frac{1}{5}$

Step 4.2

List all of the points.

$(\frac{2}{5}, - \frac{1}{5})$

Step 5