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

  • question_answer
    In  the expansion of  \[2{{\log }_{e}}x-{{\log }_{e}}(x+1)-{{\log }_{e}}(x-1)\], the coefficient of \[{{x}^{-4}}\] is

    A) 1/2

    B) \[-1\]

    C) 1

    D) None of these

    Correct Answer: A

    Solution :

    \[2{{\log }_{e}}x-{{\log }_{e}}\left\{ \left( 1+\frac{1}{x} \right)x \right\}-{{\log }_{e}}\left\{ \left( 1-\frac{1}{x} \right)x \right\}\] \[=2{{\log }_{e}}x-\left\{ {{\log }_{e}}\left( 1+\frac{1}{x} \right)+{{\log }_{e}}x \right\}\]\[-\left\{ {{\log }_{e}}\left( 1-\frac{1}{x} \right)+{{\log }_{e}}x \right\}\]      \[=-\left\{ {{\log }_{e}}\left( 1+\frac{1}{x} \right)+{{\log }_{e}}\left( 1-\frac{1}{x} \right) \right\}=2\left\{ \frac{1}{2{{x}^{2}}}+\frac{1}{4{{x}^{4}}}+....... \right\}\] The coefficient of\[{{x}^{-4}}=2.\frac{1}{4}=\frac{1}{2}\].


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