Algebra Examples

$f (x) = \log_{9} (x)$

Step 1

Write $f (x) = \log_{9} (x)$ as an equation.

$y = \log_{9} (x)$

Step 2

Interchange the variables.

$x = \log_{9} (y)$

Step 3

Solve for $y$ .

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Step 3.1

Rewrite the equation as $\log_{9} (y) = x$ .

$\log_{9} (y) = x$

Step 3.2

Rewrite $\log_{9} (y) = x$ in exponential form using the definition of a logarithm. If $x$ and $b$ are positive real numbers and $b \neq 1$ , then $\log_{b} (x) = y$ is equivalent to $b^{y} = x$ .

$9^{x} = y$

Step 3.3

Rewrite the equation as $y = 9^{x}$ .

$y = 9^{x}$

Step 4

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

$f^{- 1} (x) = 9^{x}$

Step 5

Verify if $f^{- 1} (x) = 9^{x}$ is the inverse of $f (x) = \log_{9} (x)$ .

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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 $f^{- 1} (\log_{9} (x))$ by substituting in the value of $f$ into $f^{- 1}$ .

$f^{- 1} (\log_{9} (x)) = 9^{\log_{9} (x)}$

Step 5.2.3

Exponentiation and log are inverse functions.

$f^{- 1} (\log_{9} (x)) = x$

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 $f (9^{x})$ by substituting in the value of $f^{- 1}$ into $f$ .

$f (9^{x}) = \log_{9} (9^{x})$

Step 5.3.3

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

$f (9^{x}) = x \log_{9} (9)$

Step 5.3.4

Logarithm base $9$ of $9$ is $1$ .

$f (9^{x}) = x \cdot 1$

Step 5.3.5

Multiply $x$ by $1$ .

$f (9^{x}) = x$

Step 5.4

Since $f^{- 1} (f (x)) = x$ and $f (f^{- 1} (x)) = x$ , then $f^{- 1} (x) = 9^{x}$ is the inverse of $f (x) = \log_{9} (x)$ .

$f^{- 1} (x) = 9^{x}$