Precalculus Examples

$f (x) = - 2 x^{3} + 6 x - 2$ , $[- 2, 2]$

Step 1

If $f$ is continuous on the interval $[a, b]$ and differentiable on $(a, b)$ , then at least one real number $c$ exists in the interval $(a, b)$ such that $f' (c) = \frac{f (b) - f a}{b - a}$ . The mean value theorem expresses the relationship between the slope of the tangent to the curve at $x = c$ and the slope of the line through the points $(a, f (a))$ and $(b, f (b))$ .

If $f (x)$ is continuous on $[a, b]$

and if $f (x)$ differentiable on $(a, b)$ ,

then there exists at least one point, $c$ in $[a, b]$ : $f' (c) = \frac{f (b) - f a}{b - a}$ .

Step 2

Check if $f (x) = - 2 x^{3} + 6 x - 2$ is continuous.

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Step 2.1

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.

Interval Notation:

$(- \infty, \infty)$

Set-Builder Notation:

${x | x \in ℝ}$

Step 2.2

$f (x)$ is continuous on $[- 2, 2]$ .

The function is continuous.

Step 3

Find the derivative.

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Step 3.1

Find the first derivative.

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Step 3.1.1

By the Sum Rule, the derivative of $- 2 x^{3} + 6 x - 2$ with respect to $x$ is $\frac{d}{d x} [- 2 x^{3}] + \frac{d}{d x} [6 x] + \frac{d}{d x} [- 2]$ .

$\frac{d}{d x} [- 2 x^{3}] + \frac{d}{d x} [6 x] + \frac{d}{d x} [- 2]$

Step 3.1.2

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

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Step 3.1.2.1

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

$- 2 \frac{d}{d x} [x^{3}] + \frac{d}{d x} [6 x] + \frac{d}{d x} [- 2]$

Step 3.1.2.2

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

$- 2 (3 x^{2}) + \frac{d}{d x} [6 x] + \frac{d}{d x} [- 2]$

Step 3.1.2.3

Multiply $3$ by $- 2$ .

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

Step 3.1.3

Evaluate $\frac{d}{d x} [6 x]$ .

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Step 3.1.3.1

Since $6$ is constant with respect to $x$ , the derivative of $6 x$ with respect to $x$ is $6 \frac{d}{d x} [x]$ .

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

Step 3.1.3.2

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

$- 6 x^{2} + 6 \cdot 1 + \frac{d}{d x} [- 2]$

Step 3.1.3.3

Multiply $6$ by $1$ .

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

Step 3.1.4

Differentiate using the Constant Rule.

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Step 3.1.4.1

Since $- 2$ is constant with respect to $x$ , the derivative of $- 2$ with respect to $x$ is $0$ .

$- 6 x^{2} + 6 + 0$

Step 3.1.4.2

Add $- 6 x^{2} + 6$ and $0$ .

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

Step 3.2

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

$- 6 x^{2} + 6$

Step 4

Find if the derivative is continuous on $(- 2, 2)$ .

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Step 4.1

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.

Interval Notation:

$(- \infty, \infty)$

Set-Builder Notation:

${x | x \in ℝ}$

Step 4.2

$f' (x)$ is continuous on $(- 2, 2)$ .

The function is continuous.

Step 5

The function is differentiable on $(- 2, 2)$ because the derivative is continuous on $(- 2, 2)$ .

The function is differentiable.

Step 6

$f (x)$ satisfies the two conditions for the mean value theorem. It is continuous on $[- 2, 2]$ and differentiable on $(- 2, 2)$ .

$f (x)$ is continuous on $[- 2, 2]$ and differentiable on $(- 2, 2)$ .

Step 7

Evaluate $f (a)$ from the interval $[- 2, 2]$ .

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Step 7.1

Replace the variable $x$ with $- 2$ in the expression.

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

Step 7.2

Simplify the result.

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Step 7.2.1

Simplify each term.

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Step 7.2.1.1

Multiply $- 2$ by ${(- 2)}^{3}$ by adding the exponents.

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Step 7.2.1.1.1

Multiply $- 2$ by ${(- 2)}^{3}$ .

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Step 7.2.1.1.1.1

Raise $- 2$ to the power of $1$ .

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

Step 7.2.1.1.1.2

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$f (- 2) = {(- 2)}^{1 + 3} + 6 (- 2) - 2$

Step 7.2.1.1.2

Add $1$ and $3$ .

$f (- 2) = {(- 2)}^{4} + 6 (- 2) - 2$

Step 7.2.1.2

Raise $- 2$ to the power of $4$ .

$f (- 2) = 16 + 6 (- 2) - 2$

Step 7.2.1.3

Multiply $6$ by $- 2$ .

$f (- 2) = 16 - 12 - 2$

Step 7.2.2

Simplify by subtracting numbers.

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Step 7.2.2.1

Subtract $12$ from $16$ .

$f (- 2) = 4 - 2$

Step 7.2.2.2

Subtract $2$ from $4$ .

$f (- 2) = 2$

Step 7.2.3

The final answer is $2$ .

$2$

Step 8

Evaluate $f (b)$ from the interval $[- 2, 2]$ .

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Step 8.1

Replace the variable $x$ with $2$ in the expression.

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

Step 8.2

Simplify the result.

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Step 8.2.1

Simplify each term.

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Step 8.2.1.1

Raise $2$ to the power of $3$ .

$f (2) = - 2 \cdot 8 + 6 (2) - 2$

Step 8.2.1.2

Multiply $- 2$ by $8$ .

$f (2) = - 16 + 6 (2) - 2$

Step 8.2.1.3

Multiply $6$ by $2$ .

$f (2) = - 16 + 12 - 2$

Step 8.2.2

Simplify by adding and subtracting.

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Step 8.2.2.1

Add $- 16$ and $12$ .

$f (2) = - 4 - 2$

Step 8.2.2.2

Subtract $2$ from $- 4$ .

$f (2) = - 6$

Step 8.2.3

The final answer is $- 6$ .

$- 6$

Step 9

Solve $- 6 x^{2} + 6 = \frac{- (f (b) + f (a))}{- (b + a)}$ for $x$ . $- 6 x^{2} + 6 = \frac{- (f (2) + f (- 2))}{- (2 - 2)}$ .

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Step 9.1

Simplify $\frac{(- 6) - (2)}{(2) - (- 2)}$ .

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Step 9.1.1

Reduce the expression by cancelling the common factors.

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Step 9.1.1.1

Cancel the common factor of $(- 6) - (2)$ and $(2) - (- 2)$ .

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Step 9.1.1.1.1

Rewrite $- 6$ as $- 1 (6)$ .

$- 6 x^{2} + 6 = \frac{- 1 (6) - (2)}{(2) - (- 2)}$

Step 9.1.1.1.2

Factor $- 1$ out of $- 1 (6) - (2)$ .

$- 6 x^{2} + 6 = \frac{- 1 (6 + 2)}{(2) - (- 2)}$

Step 9.1.1.1.3

Factor $2$ out of $- 1 (6 + 2)$ .

$- 6 x^{2} + 6 = \frac{2 (- 1 (3 + 1))}{2 - (- 2)}$

Step 9.1.1.1.4

Cancel the common factors.

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Step 9.1.1.1.4.1

Factor $2$ out of $2$ .

$- 6 x^{2} + 6 = \frac{2 (- 1 (3 + 1))}{2 (1) - (- 2)}$

Step 9.1.1.1.4.2

Factor $2$ out of $- (- 2)$ .

$- 6 x^{2} + 6 = \frac{2 (- 1 (3 + 1))}{2 (1) + 2 (- (- 1))}$

Step 9.1.1.1.4.3

Factor $2$ out of $2 (1) + 2 (- (- 1))$ .

$- 6 x^{2} + 6 = \frac{2 (- 1 (3 + 1))}{2 (1 - (- 1))}$

Step 9.1.1.1.4.4

Cancel the common factor.

$- 6 x^{2} + 6 = \frac{2 (- 1 (3 + 1))}{2 (1 - (- 1))}$

Step 9.1.1.1.4.5

Rewrite the expression.

$- 6 x^{2} + 6 = \frac{- 1 (3 + 1)}{1 - (- 1)}$

Step 9.1.1.2

Add $3$ and $1$ .

$- 6 x^{2} + 6 = \frac{- 1 \cdot 4}{1 - (- 1)}$

Step 9.1.2

Simplify the denominator.

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Step 9.1.2.1

Multiply $- 1$ by $- 1$ .

$- 6 x^{2} + 6 = \frac{- 1 \cdot 4}{1 + 1}$

Step 9.1.2.2

Add $1$ and $1$ .

$- 6 x^{2} + 6 = \frac{- 1 \cdot 4}{2}$

Step 9.1.3

Simplify the expression.

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Step 9.1.3.1

Multiply $- 1$ by $4$ .

$- 6 x^{2} + 6 = \frac{- 4}{2}$

Step 9.1.3.2

Divide $- 4$ by $2$ .

$- 6 x^{2} + 6 = - 2$

Step 9.2

Move all terms not containing $x$ to the right side of the equation.

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Step 9.2.1

Subtract $6$ from both sides of the equation.

$- 6 x^{2} = - 2 - 6$

Step 9.2.2

Subtract $6$ from $- 2$ .

$- 6 x^{2} = - 8$

Step 9.3

Divide each term in $- 6 x^{2} = - 8$ by $- 6$ and simplify.

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Step 9.3.1

Divide each term in $- 6 x^{2} = - 8$ by $- 6$ .

$\frac{- 6 x^{2}}{- 6} = \frac{- 8}{- 6}$

Step 9.3.2

Simplify the left side.

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Step 9.3.2.1

Cancel the common factor of $- 6$ .

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Step 9.3.2.1.1

Cancel the common factor.

$\frac{- 6 x^{2}}{- 6} = \frac{- 8}{- 6}$

Step 9.3.2.1.2

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

$x^{2} = \frac{- 8}{- 6}$

Step 9.3.3

Simplify the right side.

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Step 9.3.3.1

Cancel the common factor of $- 8$ and $- 6$ .

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Step 9.3.3.1.1

Factor $- 2$ out of $- 8$ .

$x^{2} = \frac{- 2 (4)}{- 6}$

Step 9.3.3.1.2

Cancel the common factors.

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Step 9.3.3.1.2.1

Factor $- 2$ out of $- 6$ .

$x^{2} = \frac{- 2 \cdot 4}{- 2 \cdot 3}$

Step 9.3.3.1.2.2

Cancel the common factor.

$x^{2} = \frac{- 2 \cdot 4}{- 2 \cdot 3}$

Step 9.3.3.1.2.3

Rewrite the expression.

$x^{2} = \frac{4}{3}$

Step 9.4

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

$x = \pm \sqrt{\frac{4}{3}}$

Step 9.5

Simplify $\pm \sqrt{\frac{4}{3}}$ .

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Step 9.5.1

Rewrite $\sqrt{\frac{4}{3}}$ as $\frac{\sqrt{4}}{\sqrt{3}}$ .

$x = \pm \frac{\sqrt{4}}{\sqrt{3}}$

Step 9.5.2

Simplify the numerator.

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Step 9.5.2.1

Rewrite $4$ as $2^{2}$ .

$x = \pm \frac{\sqrt{2^{2}}}{\sqrt{3}}$

Step 9.5.2.2

Pull terms out from under the radical, assuming positive real numbers.

$x = \pm \frac{2}{\sqrt{3}}$

Step 9.5.3

Multiply $\frac{2}{\sqrt{3}}$ by $\frac{\sqrt{3}}{\sqrt{3}}$ .

$x = \pm \frac{2}{\sqrt{3}} \cdot \frac{\sqrt{3}}{\sqrt{3}}$

Step 9.5.4

Combine and simplify the denominator.

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Step 9.5.4.1

Multiply $\frac{2}{\sqrt{3}}$ by $\frac{\sqrt{3}}{\sqrt{3}}$ .

$x = \pm \frac{2 \sqrt{3}}{\sqrt{3} \sqrt{3}}$

Step 9.5.4.2

Raise $\sqrt{3}$ to the power of $1$ .

$x = \pm \frac{2 \sqrt{3}}{{\sqrt{3}}^{1} \sqrt{3}}$

Step 9.5.4.3

Raise $\sqrt{3}$ to the power of $1$ .

$x = \pm \frac{2 \sqrt{3}}{{\sqrt{3}}^{1} {\sqrt{3}}^{1}}$

Step 9.5.4.4

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$x = \pm \frac{2 \sqrt{3}}{{\sqrt{3}}^{1 + 1}}$

Step 9.5.4.5

Add $1$ and $1$ .

$x = \pm \frac{2 \sqrt{3}}{{\sqrt{3}}^{2}}$

Step 9.5.4.6

Rewrite ${\sqrt{3}}^{2}$ as $3$ .

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Step 9.5.4.6.1

Use $\sqrt[n]{a^{x}} = a^{\frac{x}{n}}$ to rewrite $\sqrt{3}$ as $3^{\frac{1}{2}}$ .

$x = \pm \frac{2 \sqrt{3}}{{(3^{\frac{1}{2}})}^{2}}$

Step 9.5.4.6.2

Apply the power rule and multiply exponents, ${(a^{m})}^{n} = a^{m n}$ .

$x = \pm \frac{2 \sqrt{3}}{3^{\frac{1}{2} \cdot 2}}$

Step 9.5.4.6.3

Combine $\frac{1}{2}$ and $2$ .

$x = \pm \frac{2 \sqrt{3}}{3^{\frac{2}{2}}}$

Step 9.5.4.6.4

Cancel the common factor of $2$ .

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Step 9.5.4.6.4.1

Cancel the common factor.

$x = \pm \frac{2 \sqrt{3}}{3^{\frac{2}{2}}}$

Step 9.5.4.6.4.2

Rewrite the expression.

$x = \pm \frac{2 \sqrt{3}}{3^{1}}$

Step 9.5.4.6.5

Evaluate the exponent.

$x = \pm \frac{2 \sqrt{3}}{3}$

Step 9.6

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

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Step 9.6.1

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

$x = \frac{2 \sqrt{3}}{3}$

Step 9.6.2

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

$x = - \frac{2 \sqrt{3}}{3}$

Step 9.6.3

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

$x = \frac{2 \sqrt{3}}{3}, - \frac{2 \sqrt{3}}{3}$

Step 10

There is a tangent line found at $x = \frac{2 \sqrt{3}}{3}$ parallel to the line that passes through the end points $a = - 2$ and $b = 2$ .

There is a tangent line at $x = \frac{2 \sqrt{3}}{3}$ parallel to the line that passes through the end points $a = - 2$ and $b = 2$

Step 11

There is a tangent line found at $x = - \frac{2 \sqrt{3}}{3}$ parallel to the line that passes through the end points $a = - 2$ and $b = 2$ .

There is a tangent line at $x = - \frac{2 \sqrt{3}}{3}$ parallel to the line that passes through the end points $a = - 2$ and $b = 2$

Step 12