Logarithmic Equation Solution Count

XAT 2025 · Quantitative Ability & Data Interpretation · Medium · 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)−log⁡a(64)log⁡a(32)−x=0\frac{x^2 \log_a(16)}{\log_a(32)} - \frac{\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 using change of base formula: 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−x−65=0  ⟹  4x2−5x−6=0\frac{4}{5} x^2 - x - \frac{6}{5} = 0 \implies 4x^2 - 5x - 6 = 0. Discriminant D=25−4(4)(−6)=121>0D = 25 - 4(4)(-6) = 121 > 0, giving 2 real roots. However, the logarithmic terms log⁡a(16),log⁡a(32)\log_a(16), \log_a(32) require base a>0a > 0 and a≠1a \neq 1. If a=1a = 1, the logarithm is undefined. Therefore, the existence of solutions depends on whether a≠1a \neq 1, i.e., depends on the value of aa.

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