Manipal Engineering Manipal Engineering Solved Paper-2008

  • question_answer
    If\[P={{x}^{3}}-\frac{1}{{{x}^{3}}}\]and\[Q=x-\frac{1}{x},\,\,x\in (0,\,\,x)\], then minimum value of\[\frac{P}{{{Q}^{2}}}\]is

    A) \[2\sqrt{3}\]                                      

    B) \[-2\sqrt{3}\]

    C)  does not exist

    D)         None of these

    Correct Answer: C

    Solution :

    Given,                 \[P={{x}^{3}}-\frac{1}{{{x}^{3}}},\,\,Q=x-\frac{1}{x}\] \[\therefore \]  \[\frac{P}{{{Q}^{2}}}=\frac{\left( x-\frac{1}{x} \right)\left( {{x}^{2}}+1+\frac{1}{{{x}^{2}}} \right)}{{{\left( x-\frac{1}{x} \right)}^{2}}}\]                 \[=\frac{{{\left( x-\frac{1}{x} \right)}^{2}}+3}{\left( x-\frac{1}{x} \right)}\]                 \[=\left( x-\frac{1}{x} \right)+\frac{3}{\left( x-\frac{1}{x} \right)}\] Clearly, the minimum does not exist.


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