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

  • question_answer
    Sum of the infinite series \[1+2+\frac{1}{2!}+\frac{2}{3!}+\frac{1}{4!}+\frac{2}{5!}+.....\] is [Roorkee 2000]

    A) \[{{e}^{2}}\]

    B) \[e+{{e}^{-1}}\]

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

    D) \[\frac{3e-{{e}^{-1}}}{2}\]

    Correct Answer: D

    Solution :

    Sum of series \[=1+2+\frac{1}{2!}+\frac{2}{3!}+\frac{1}{4!}+\frac{2}{5!}+.....\] \[=\left( 1+\frac{1}{2!}+\frac{1}{4!}+.... \right)+2\left( 1+\frac{1}{3!}+\frac{1}{5!}+..... \right)\] = \[\frac{e+{{e}^{-1}}}{2}+2.\frac{e-{{e}^{-1}}}{2}=\frac{3e-{{e}^{-1}}}{2}\].


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