Precalculus Examples

$f (x) = e^{x + 4}$

Step 1

Write $f (x) = e^{x + 4}$ as an equation.

$y = e^{x + 4}$

Step 2

Interchange the variables.

$x = e^{y + 4}$

Step 3

Solve for $y$ .

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Step 3.1

Rewrite the equation as $e^{y + 4} = x$ .

$e^{y + 4} = x$

Step 3.2

Take the natural logarithm of both sides of the equation to remove the variable from the exponent.

$\ln (e^{y + 4}) = \ln (x)$

Step 3.3

Expand the left side.

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Step 3.3.1

Expand $\ln (e^{y + 4})$ by moving $y + 4$ outside the logarithm.

$(y + 4) \ln (e) = \ln (x)$

Step 3.3.2

The natural logarithm of $e$ is $1$ .

$(y + 4) \cdot 1 = \ln (x)$

Step 3.3.3

Multiply $y + 4$ by $1$ .

$y + 4 = \ln (x)$

Step 3.4

Subtract $4$ from both sides of the equation.

$y = \ln (x) - 4$

Step 4

Replace $y$ with $f^{- 1} (x)$ to show the final answer.

$f^{- 1} (x) = \ln (x) - 4$

Step 5

Verify if $f^{- 1} (x) = \ln (x) - 4$ is the inverse of $f (x) = e^{x + 4}$ .

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Step 5.1

To verify the inverse, check if $f^{- 1} (f (x)) = x$ and $f (f^{- 1} (x)) = x$ .

Step 5.2

Evaluate $f^{- 1} (f (x))$ .

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Step 5.2.1

Set up the composite result function.

$f^{- 1} (f (x))$

Step 5.2.2

Evaluate $f^{- 1} (e^{x + 4})$ by substituting in the value of $f$ into $f^{- 1}$ .

$f^{- 1} (e^{x + 4}) = \ln (e^{x + 4}) - 4$

Step 5.2.3

Simplify each term.

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Step 5.2.3.1

Use logarithm rules to move $x + 4$ out of the exponent.

$f^{- 1} (e^{x + 4}) = (x + 4) \ln (e) - 4$

Step 5.2.3.2

The natural logarithm of $e$ is $1$ .

$f^{- 1} (e^{x + 4}) = (x + 4) \cdot 1 - 4$

Step 5.2.3.3

Multiply $x + 4$ by $1$ .

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

Step 5.2.4

Combine the opposite terms in $x + 4 - 4$ .

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Step 5.2.4.1

Subtract $4$ from $4$ .

$f^{- 1} (e^{x + 4}) = x + 0$

Step 5.2.4.2

Add $x$ and $0$ .

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

Step 5.3

Evaluate $f (f^{- 1} (x))$ .

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Step 5.3.1

Set up the composite result function.

$f (f^{- 1} (x))$

Step 5.3.2

Evaluate $f (\ln (x) - 4)$ by substituting in the value of $f^{- 1}$ into $f$ .

$f (\ln (x) - 4) = e^{(\ln (x) - 4) + 4}$

Step 5.3.3

Combine the opposite terms in $(\ln (x) - 4) + 4$ .

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Step 5.3.3.1

Add $- 4$ and $4$ .

$f (\ln (x) - 4) = e^{\ln (x) + 0}$

Step 5.3.3.2

Add $\ln (x)$ and $0$ .

$f (\ln (x) - 4) = e^{\ln (x)}$

Step 5.3.4

Exponentiation and log are inverse functions.

$f (\ln (x) - 4) = x$

Step 5.4

Since $f^{- 1} (f (x)) = x$ and $f (f^{- 1} (x)) = x$ , then $f^{- 1} (x) = \ln (x) - 4$ is the inverse of $f (x) = e^{x + 4}$ .

$f^{- 1} (x) = \ln (x) - 4$