Logarithmic Simplification

XAT 2019 · Quantitative Ability & Data Interpretation · Hard · Algebra

log⁡(97−563)log⁡7+43\frac{\log(97-56\sqrt{3})}{\log\sqrt{7+4\sqrt{3}}} equals which of the following?

  1. A.

    None of the others

  2. B.

    -2

  3. C.

    -4

  4. D.

    -3

  5. E.

    -5

Answer

C

Explanation

Note that (7−43)2=49+48−563=97−563(7 - 4\sqrt{3})^2 = 49 + 48 - 56\sqrt{3} = 97 - 56\sqrt{3}. So 97−563=(7−43)297 - 56\sqrt{3} = (7 - 4\sqrt{3})^2. Also 7−43=17+43=(7+43)−17 - 4\sqrt{3} = \frac{1}{7 + 4\sqrt{3}} = (7 + 4\sqrt{3})^{-1}. Thus 97−563=(7+43)−297 - 56\sqrt{3} = (7 + 4\sqrt{3})^{-2}. Denominator: log⁡7+43=12log⁡(7+43)\log\sqrt{7 + 4\sqrt{3}} = \frac{1}{2} \log(7 + 4\sqrt{3}). Numerator: log⁡((7+43)−2)=−2log⁡(7+43)\log((7 + 4\sqrt{3})^{-2}) = -2 \log(7 + 4\sqrt{3}). Ratio = −2log⁡(7+43)12log⁡(7+43)=−4\frac{-2 \log(7 + 4\sqrt{3})}{\frac{1}{2} \log(7 + 4\sqrt{3})} = -4.

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