JEE Main & Advanced Mathematics Sequence & Series Question Bank Logarithmic series

  • question_answer
    The sum to infinity of the given series \[\frac{1}{n}-\frac{1}{2{{n}^{2}}}+\frac{1}{3{{n}^{3}}}-\frac{1}{4{{n}^{4}}}+....\]  is [MP PET 1994]

    A) \[{{\log }_{e}}\left( \frac{n+1}{n} \right)\]

    B) \[{{\log }_{e}}\left( \frac{n}{n+1} \right)\]

    C) \[{{\log }_{e}}\left( \frac{n-1}{n} \right)\]

    D) \[{{\log }_{e}}\left( \frac{n}{n-1} \right)\]

    Correct Answer: A

    Solution :

    \[\frac{1}{n}-\frac{1}{2{{n}^{2}}}+\frac{1}{3{{n}^{3}}}-\frac{1}{4{{n}^{4}}}+....\] \[=\frac{1}{n}-\frac{{{(1/n)}^{2}}}{2}+\frac{{{(1/n)}^{3}}}{3}-\frac{{{(1/n)}^{4}}}{4}+....\] \[={{\log }_{e}}\left( 1+\frac{1}{n} \right)={{\log }_{e}}\left( \frac{n+1}{n} \right)\].


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