Number of Real Roots in Logarithmic Equation

XAT 2025 · Quantitative Ability & Data Interpretation · Hard · Logarithms

For how many distinct real values of xx does the equation below hold true? (Consider a>0a > 0.) x2log⁡a(16)log⁡a(32)−xlog⁡a(64)log⁡a(32)−x=0\frac{x^2 \log_a(16)}{\log_a(32)} - \frac{x \log_a(64)}{\log_a(32)} - x = 0

  1. A.

    1

  2. B.

    Infinitely many

  3. C.

    2

  4. D.

    0

  5. E.

    Depends on the value of a

Answer

E

Explanation

Simplify the ratios using base change rule log⁡amlog⁡an=log⁡nm\frac{\log_a m}{\log_a n} = \log_n m:

  • log⁡a(16)log⁡a(32)=log⁡32(16)=45\frac{\log_a(16)}{\log_a(32)} = \log_{32}(16) = \frac{4}{5}.
  • log⁡a(64)log⁡a(32)=log⁡32(64)=65\frac{\log_a(64)}{\log_a(32)} = \log_{32}(64) = \frac{6}{5}. The equation becomes: 45x2−65x−x=0  ⟹  45x2−115x=0  ⟹  x(45x−115)=0\frac{4}{5}x^2 - \frac{6}{5}x - x = 0 \implies \frac{4}{5}x^2 - \frac{11}{5}x = 0 \implies x\left(\frac{4}{5}x - \frac{11}{5}\right) = 0. So x=0x = 0 or x=114x = \frac{11}{4}. However, for log⁡a(x)\log_a(x) terms to be defined, base aa must satisfy a>0a > 0 and a≠1a \neq 1. If a=1a = 1, the terms log⁡a(32)\log_a(32) are undefined. Thus the valid roots depend on whether aa is valid (a≠1a \ne 1). Therefore, the answer depends on the value of aa.

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