Question 109

Mathematics Function and Relation Hard

If \(f(x)=\lim _{{x}\rightarrow0}\, \frac{{6}^x-{3}^x-{2}^x+1}{\log _e9(1-\cos x)}\) is a real number,
then \(\lim _{{x}\rightarrow0}\, f(x)\)

(A) 2
(B) 3
(C) \(\log _e2\)
(D) \(\log _e3\)
✓ Correct Answer: Option C Verified Solution

📘 Step-by-Step Solution & Detailed Explanation:

\[ f(x) = \lim_{{x}\rightarrow 0} \frac{{6}^x - {3}^x - {2}^x + 1}{\log_e 9 (1 - \cos x)}, \] we use Taylor series and L'Hôpital's Rule. For small \( x \), \( 1 - \cos x \approx \frac{x^2}{2} \), and \( a^x \approx 1 + x \ln a \). The numerator becomes: \[ 6^x - 3^x - 2^x + 1 \approx x (\ln 6 - \ln 3 - \ln 2) + \frac{x^2}{2} \left((\ln 6)^2 - (\ln 3)^2 - (\ln 2)^2\right). \] Since \( \ln 6 = \ln 2 + \ln 3 \), the first-order term vanishes, leaving: \[ \frac{x^2}{2} \left((\ln 6)^2 - (\ln 3)^2 - (\ln 2)^2\right). \] Substituting into the limit: \[ f(x) = \frac{(\ln 6)^2 - (\ln 3)^2 - (\ln 2)^2}{2 \log_e 3}. \] Simplify using \( (\ln 6)^2 = (\ln 2 + \ln 3)^2 \): \[ f(x) = \frac{2 \ln 2 \ln 3}{2 \log_e 3} = \ln 2. \]

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