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

  • question_answer
    In  the expansion of \[{{({{e}^{x}}-1)}^{2}}\], the coefficient of \[{{x}^{4}}\] will be

    A) 1/12

    B) 7/12

    C) 5/12

    D) None of these

    Correct Answer: B

    Solution :

    \[{{e}^{2x}}-2{{e}^{x}}+1=1+\frac{2x}{1\ !}+\frac{{{(2x)}^{2}}}{2\ !}+\frac{{{(2x)}^{3}}}{3\ !}+\frac{{{(2x)}^{4}}}{4\ !}+......\]\[-2\left\{ 1+\frac{x}{1\ !}+\frac{{{x}^{2}}}{2\ !}+\frac{{{x}^{3}}}{3\ !}+\frac{{{x}^{4}}}{4\ !}+..... \right\}+1\] \[\therefore \] The coefficient of       \[{{x}^{4}}\]\[=\frac{{{2}^{4}}}{4\ !}-2.\frac{1}{4\ !}=\frac{2}{4\ !}({{2}^{3}}-1)=\frac{7}{12}\].


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