JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Maxima and Minima

  • question_answer
    On [1, e] the greatest value of \[{{x}^{2}}\log x\]   [AMU 2002]

    A)            \[{{e}^{2}}\]

    B)            \[\frac{1}{e}\log \frac{1}{\sqrt{e}}\]

    C)            \[{{e}^{2}}\log \sqrt{e}\]

    D)            None of these

    Correct Answer: A

    Solution :

               \[f(x)={{x}^{2}}\log x\] Þ \[{f}'(x)=(2\log x+1)x\]            Now \[{f}'(x)=0\] Þ \[x={{e}^{-1/2}},\,0\]            \[\because \]  \[0<{{e}^{-1/2}}<1\]            \[\because \] None of these critical points lies in the interval [1, e]            \ So we only complete the value of \[f(x)\] at the end points 1 and e. We have \[f(1)=0,\,f(e)={{e}^{2}}\]            \  Greatest value = \[{{e}^{2}}.\]


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