JEE Main & Advanced JEE Main Online Paper (Held On 08 April 2018)

  • question_answer
    Let \[f(x)={{x}^{2}}+\frac{1}{{{x}^{2}}}\]and \[g(x)=x-\frac{1}{x},\]\[x\in R-\{-1,0,1\}.\] If \[h(x)=\frac{f(x)}{g(x)}\], then the local minimum value of \[h(x)\] is: [JEE Main Online 08-04-2018]

    A)  \[-2\sqrt{2}\]              

    B)  \[2\sqrt{2}\]

    C)  3                                

    D)  -3

    Correct Answer: B

    Solution :

    \[F(x)={{x}^{2}}+\frac{1}{{{x}^{2}}},\,\,g(x)=x-\frac{1}{x}\] \[h(x)=\frac{F(x)}{g(x)}\] \[h(x)=\frac{{{x}^{2}}+\frac{1}{{{x}^{2}}}}{x-\frac{1}{x}}\] \[f(\alpha )=\frac{{{\alpha }^{2}}+2}{\alpha }\] \[f(\alpha )=\alpha +\frac{2}{\alpha }\] \[F(\alpha )={{\left( \sqrt{\alpha }-\sqrt{\frac{2}{\alpha }} \right)}^{2}}+2\sqrt{2}\] Minimum value will be \[2\sqrt{2}\] At \[\sqrt{\alpha }-\sqrt{\frac{2}{\alpha }}=0\] \[\alpha =\sqrt{2}\]which is possible as \[x-\frac{1}{x}\in R\]


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