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

  • question_answer
     The minimum value of \[\frac{\log x}{x}\] in the interval \[[2,\,\infty )\] is                                                                       [Roorkee 1999]

    A)            \[\frac{\log 2}{2}\]

    B)            Zero

    C)            \[\frac{1}{e}\]

    D)            Does not exist

    Correct Answer: D

    Solution :

               Let \[y=\frac{\log x}{x}\] Þ             \[\frac{dy}{dx}=\frac{x.\frac{1}{x}-\log x}{{{x}^{2}}}\]\[=\frac{1-\log x}{{{x}^{2}}}\]            Put \[\frac{dy}{dx}=0\Rightarrow \frac{1-\log x}{{{x}^{2}}}=0\]            Þ  \[1-\log x=0\] Þ \[x=e\] and \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=\frac{-3x+2x\log x}{{{x}^{4}}}\]            At   \[x=e\], \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=\frac{1}{-{{e}^{3}}}<0\]                    \ In [2, ¥) the function \[{{p}^{2}}=q\] will be maximum and minimum value does not exist.


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