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

  • question_answer
    The sum of \[\frac{1}{2}+\frac{1}{3}.\frac{1}{{{2}^{3}}}+\frac{1}{5}.\frac{1}{{{2}^{5}}}+.....\infty \] is [MP PET 1991]

    A) \[{{\log }_{e}}\sqrt{\frac{3}{2}}\]

    B) \[{{\log }_{e}}\sqrt{3}\]

    C) \[{{\log }_{e}}\sqrt{\frac{1}{2}}\]

    D) \[{{\log }_{e}}3\]

    Correct Answer: B

    Solution :

    Sum of \[\frac{1}{2}+\frac{1}{3}.\frac{1}{{{2}^{.3}}}+\frac{1}{5}.\frac{1}{{{2}^{5}}}+....\infty \] = \[\frac{1}{2}\left[ 1+\frac{1}{3}.\frac{1}{{{2}^{2}}}+\frac{1}{5}.\frac{1}{{{2}^{4}}}+....\infty  \right]\]\[=\frac{1}{2}.{{\log }_{e}}\left( \frac{1+\frac{1}{2}}{1-\frac{1}{2}} \right)\] = \[\frac{1}{2}.{{\log }_{e}}\left( \frac{3/2}{1/2} \right)=\frac{1}{2}.{{\log }_{e}}(3)={{\log }_{e}}\sqrt{3}\].


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