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

  • question_answer
    \[1+\frac{1+2}{2\,!}+\frac{1+2+3}{3\,!}+\frac{1+2+3+4}{4\,!}+....\infty =\] [Roorkee 1999; MP PET 2003]

    A) \[e\]

    B) \[3\,e\]

    C)  \[e/2\]

    D)   \[3e/2\]

    Correct Answer: D

    Solution :

    \[{{T}_{n}}=\frac{\Sigma n}{n\,!}=\frac{n(n+1)}{2\,(n)\,!}\]    \[=\frac{1}{2}\left[ \frac{(n+1)}{(n-1)!} \right]=\frac{1}{2}\left[ \frac{n-1}{(n-1)!}+\frac{2}{(n-1)!} \right]\]        \[=\frac{1}{2}\left[ \frac{1}{(n-2)!}+\frac{2}{(n-1)!} \right]=\frac{(e+2e)}{2}=\frac{3e}{2}\].


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