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Find Inverse of Logarithmic Functions

Examples with detailed solutions on how to find the inverse of logarithmic functions and determine their domain and range.

Example 1

Find the inverse function, its domain, and range of the function:

\[ f(x) = \ln(x - 2) \]
View Step-by-Step Solution
  • Domain of \(f\): \(x - 2 > 0 \Rightarrow x > 2\), so \(\text{Domain}(f) = (2, +\infty)\) and \(\text{Range}(f) = (-\infty, +\infty)\).
  • Write the function as an equation: \[ y = \ln(x - 2) \]
  • Convert to exponential form: \[ x - 2 = e^y \]
  • Solve for \(x\): \[ x = 2 + e^y \]
  • Swap \(x\) and \(y\) to get the inverse function: \[ f^{-1}(x) = 2 + e^x \]

Conclusion: \(\text{Domain}(f^{-1}) = (-\infty, +\infty)\), \(\text{Range}(f^{-1}) = (2, +\infty)\).

Example 2

Find the inverse, its domain, and range of:

\[ f(x) = 3\ln(4x - 6) - 2 \]
View Step-by-Step Solution
  • Domain of \(f\): \(4x - 6 > 0 \Rightarrow x > \frac{3}{2}\), so \(\text{Domain}(f) = \left(\frac{3}{2}, +\infty\right)\), \(\text{Range}(f) = (-\infty, +\infty)\).
  • Equation form and isolation: \[ y = 3\ln(4x - 6) - 2 \implies \ln(4x - 6) = \frac{y + 2}{3} \]
  • Exponential form: \[ 4x - 6 = e^{(y + 2)/3} \]
  • Solve for \(x\): \[ 4x = e^{(y + 2)/3} + 6 \implies x = \frac{1}{4} e^{(y + 2)/3} + \frac{3}{2} \]
  • Swap variables for the inverse function: \[ f^{-1}(x) = \frac{1}{4} e^{(x + 2)/3} + \frac{3}{2} \]

Conclusion: \(\text{Domain}(f^{-1}) = (-\infty, +\infty)\), \(\text{Range}(f^{-1}) = \left(\frac{3}{2}, +\infty\right)\).

Example 3

Find the inverse, its domain, and range of:

\[ f(x) = -\ln(x^2 - 4) - 5, \quad x < -2 \]
View Step-by-Step Solution
  • Domain and Range of \(f\): \(\text{Domain}(f) = (-\infty, -2)\), \(\text{Range}(f) = (-\infty, +\infty)\).
  • Equation form: \[ y = -\ln(x^2 - 4) - 5 \implies \ln(x^2 - 4) = -y - 5 \]
  • Exponential form: \[ x^2 - 4 = e^{-y - 5} \implies x^2 = e^{-y - 5} + 4 \implies x = \pm \sqrt{e^{-y - 5} + 4} \]
  • Select root using domain restriction (\(x < -2\)): \[ x = - \sqrt{e^{-y - 5} + 4} \]
  • Inverse function: \[ f^{-1}(x) = - \sqrt{e^{-x - 5} + 4} \]

Conclusion: \(\text{Domain}(f^{-1}) = (-\infty, +\infty)\), \(\text{Range}(f^{-1}) = (-\infty, -2)\).


Exercises

Find the inverse, domain, and range of:

  1. \[ f(x) = - \ln(-x + 4) - 6 \]
  2. \[ g(x) = \ln(x^2 - 1) - 3, \quad x > 1 \]

Answers and Solutions

  1. \( f^{-1}(x) = - e^{-x - 6} + 4 \)
    Domain: \((-\infty, +\infty)\), Range: \((-\infty, 4)\)
  2. \( g^{-1}(x) = \sqrt{1 + e^{x + 3}} \)
    Domain: \((-\infty, +\infty)\), Range: \((1, +\infty)\)

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