Logarithmic Equation Real Solutions

XAT 2024 · Quantitative Ability & Data Interpretation · Easy · Algebra

Consider the equation log⁡5(x−2)=2log⁡25(2x−4)\log_5(x - 2) = 2 \log_{25}(2x - 4), where xx is a real number. For how many different values of xx does the given equation hold?

  1. A.

    0

  2. B.

    1

  3. C.

    2

  4. D.

    4

  5. E.

    Infinitely many

Answer

A

Explanation

RHS =2log⁡25(2x−4)=2⋅12log⁡5(2x−4)=log⁡5(2x−4)= 2 \log_{25}(2x - 4) = 2 \cdot \frac{1}{2} \log_5(2x - 4) = \log_5(2x - 4). So log⁡5(x−2)=log⁡5(2x−4)\log_5(x - 2) = \log_5(2x - 4). Equating arguments: x−2=2x−4  ⟹  x=2x - 2 = 2x - 4 \implies x = 2.

Checking domain: For x=2x = 2, x−2=0x - 2 = 0, but the argument of a logarithm must be strictly positive (x−2>0x - 2 > 0). Thus x=2x = 2 is extraneous and invalid. Therefore, there are 0 valid real solutions.

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