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

  • question_answer
    The coefficient of \[{{x}^{n}}\] in the expansion of \[{{\log }_{a}}(1+x)\]  is

    A) \[\frac{{{(-1)}^{n-1}}}{n}\]

    B) \[\frac{{{(-1)}^{n-1}}}{n}{{\log }_{a}}e\]

    C) \[\frac{{{(-1)}^{n-1}}}{n}{{\log }_{e}}a\]

    D) \[\frac{{{(-1)}^{n}}}{n}{{\log }_{a}}e\]

    Correct Answer: B

    Solution :

    We have \[{{\log }_{a}}(1+x)={{\log }_{e}}(1+x).{{\log }_{a}}e\]\[={{\log }_{a}}e\left( \sum\limits_{n=1}^{\infty }{{{(-1)}^{n-1}}\frac{{{x}^{n}}}{n}} \right)\] Therefore coefficient of \[{{x}^{n}}\]in \[{{\log }_{a}}(1+x)\] is\[\frac{{{(-1)}^{n-1}}}{n}{{\log }_{a}}e\].


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