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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Functions

NDA 2025 · Mathematics · Functions

For the following two (02) items: Consider the function f(x) = 10^x...

For the following two (02) items:
Consider the function f(x)=10x−10−x10x+10−xf(x) = \dfrac{10^x - 10^{-x}}{10^x + 10^{-x}}.

What is the inverse of the function?

Options

  1. Alog⁡10(2x−1)\log_{10}(2x - 1)
  2. B12log⁡10(2x−1)\dfrac{1}{2}\log_{10}(2x - 1)
  3. C14log⁡10(2x2−x)\dfrac{1}{4}\log_{10} \left(\dfrac{2x}{2 - x}\right)
  4. D12log⁡10(1+x1−x)\dfrac{1}{2}\log_{10} \left(\dfrac{1 + x}{1 - x}\right)

Correct answer

D. 12log⁡10(1+x1−x)\dfrac{1}{2}\log_{10} \left(\dfrac{1 + x}{1 - x}\right)

Step-by-step solution

Let y=f(x)=10x−10−x10x+10−xy = f(x) = \dfrac{10^x - 10^{-x}}{10^x + 10^{-x}}

Multiplying the numerator and the denominator by 10x10^x, we get:

y=102x−1102x+1y = \dfrac{10^{2x} - 1}{10^{2x} + 1}

Applying componendo and dividendo:

1+y1−y=102x+1+102x−1102x+1−(102x−1)\dfrac{1 + y}{1 - y} = \dfrac{10^{2x} + 1 + 10^{2x} - 1}{10^{2x} + 1 - (10^{2x} - 1)}

1+y1−y=2⋅102x2\dfrac{1 + y}{1 - y} = \dfrac{2 \cdot 10^{2x}}{2}

102x=1+y1−y10^{2x} = \dfrac{1 + y}{1 - y}

Taking logarithm to the base 1010 on both sides:

2x=log⁡10(1+y1−y)2x = \log_{10}\left(\dfrac{1 + y}{1 - y}\right)

x=12log⁡10(1+y1−y)x = \dfrac{1}{2}\log_{10}\left(\dfrac{1 + y}{1 - y}\right)

Replacing yy with xx to find the inverse function:

f−1(x)=12log⁡10(1+x1−x)f^{-1}(x) = \dfrac{1}{2}\log_{10}\left(\dfrac{1 + x}{1 - x}\right)

Answer: $\dfrac{1}{2}\log_{10}\left(\dfrac{1 + x}{1 - x}\right)
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