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

  • question_answer
    The sum of the series \[\frac{1}{2\,!}-\frac{1}{3\,!}+\frac{1}{4\,!}-.....\] is [DCE 2002]

    A) e

    B) \[{{e}^{-\,\frac{1}{2}}}\]

    C) \[{{e}^{-\,2}}\]

    D) None of these

    Correct Answer: D

    Solution :

    We know, \[{{e}^{x}}=1+x+\frac{{{x}^{2}}}{2!}+\frac{{{x}^{3}}}{3!}+\frac{{{x}^{4}}}{4!}+....\] Put \[x=-1\]\[\Rightarrow {{e}^{-1}}=1-1+\frac{1}{2!}-\frac{1}{3!}+\frac{1}{4!}-\frac{1}{5!}+.....\]                  Þ \[{{e}^{-1}}=\frac{1}{2!}-\frac{1}{3!}+\frac{1}{4!}-\frac{1}{5!}+.....\]


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