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

  • question_answer
    \[\left( \frac{a-b}{a} \right)+\frac{1}{2}{{\left( \frac{a-b}{a} \right)}^{2}}+\frac{1}{3}\,{{\left( \frac{a-b}{a} \right)}^{3}}+.....=\] [MNR 1979; MP PET 1990; UPSEAT 2001, 02; AMU 2005]

    A) \[{{\log }_{e}}(a-b)\]

    B) \[{{\log }_{e}}\left( \frac{a}{b} \right)\]

    C) \[{{\log }_{e}}\left( \frac{b}{a} \right)\]

    D)  \[{{e}^{\left( \frac{a-b}{a} \right)}}\]

    Correct Answer: B

    Solution :

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


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