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

  • question_answer
    \[1+\frac{1+3}{2\,!}+\frac{1+3+5}{3\,!}+\frac{1+3+5+7}{4\,!}+.......\infty =\]

    A) \[e/2\]

    B) \[e\]

    C) \[2\,e\]

    D) \[3e\]

    Correct Answer: C

    Solution :

    \[{{T}_{n}}=\frac{1+3+5+.......+(2n-1)}{n\ !}=\frac{\frac{n}{2}[2\ .1+(n-1)2]}{n\ !}\]           \[=\frac{n-1+1}{(n-1)\ !}=\frac{1}{(n-2)\ !}+\frac{1}{(n-1)\ !}\] \[\Rightarrow S=\sum\limits_{n=1}^{\infty }{{{T}_{n}}}=e+e=2e\].


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