KVPY Sample Paper KVPY Stream-SX Model Paper-13

  • question_answer
    The sum of all the roots of the equation \[\sin \left( \pi {{\log }_{3}}\left( \frac{1}{x} \right) \right)=0\]in \[(0,2\pi )\]

    A) \[\frac{3}{2}\]

    B) \[4\]

    C) \[\frac{9}{2}\]

    D) \[\frac{13}{3}\]

    Correct Answer: C

    Solution :

    Given, \[\sin \left[ \pi {{\log }_{3}}\left( \frac{1}{x} \right) \right]=0\]
    \[\Rightarrow \]   \[\pi {{\log }_{3}}\left( \frac{1}{x} \right)=n\pi \]
    \[\Rightarrow \]   \[{{\log }_{3}}\left( \frac{1}{x} \right)=n\]
    \[\Rightarrow \]               \[x={{3}^{-n}}\]
    Sum of roots \[=3+1+\frac{1}{3}+\frac{1}{{{3}^{2}}}+\frac{1}{{{3}^{3}}}+...+\infty \]
    \[[\because x\in (0,2\pi )]\]
    \[=4+\frac{1}{3}\left( \frac{1}{1-\frac{1}{3}} \right)=4+\frac{1}{2}=\frac{9}{2}\]


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