JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    The nth derivative of \[x{{e}^{x}}\] vanishes when [AMU 1999]

    A)            \[x=0\]

    B)            \[x=-1\]

    C)            \[x=-n\]

    D)            \[x=n\]

    Correct Answer: C

    Solution :

               \[f(x)=x{{e}^{x}}\]            \[{f}'(x)={{e}^{x}}+x{{e}^{x}}\]            \[{f}''(x)={{e}^{x}}+{{e}^{x}}+x{{e}^{x}}=2{{e}^{x}}+x{{e}^{x}}\]            \[{f}'''(x)=2{{e}^{x}}+{{e}^{x}}+x{{e}^{x}}=3{{e}^{x}}+x{{e}^{x}}\]            ?????????????????            ?????????????????            \[{{f}^{n}}(x)=n{{e}^{x}}+x{{e}^{x}}\]. Now, \[{{f}^{n}}(x)=0\]            Þ \[n{{e}^{x}}+x{{e}^{x}}=0\] Þ\[x=-n\].


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