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

  • question_answer
    The coefficient of  \[{{x}^{r}}\] in the expansion of \[1+\frac{a+bx}{1\,!}+\frac{{{(a+bx)}^{2}}}{2\,!}+.....+\frac{{{(a+bx)}^{n}}}{n\,!}+.....\] is [MP PET 1989]

    A) \[\frac{{{(a+b)}^{r}}}{r\,!}\]

    B) \[\frac{{{b}^{r}}}{r\,!}\]

    C) \[\frac{{{e}^{a}}{{b}^{r}}}{r\,!}\]

    D) \[{{e}^{a+{{b}^{r}}}}\]

    Correct Answer: C

    Solution :

    \[S={{e}^{a+bx}}={{e}^{a}}.{{e}^{bx}}={{e}^{a}}\left\{ 1+\frac{bx}{1\ !}+\frac{{{(bx)}^{2}}}{2\ !}+..... \right\}\] The coefficient of\[{{x}^{r}}={{e}^{a}}.\frac{{{b}^{r}}}{r\ !}\].


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