KVPY Sample Paper KVPY Stream-SX Model Paper-13

  • question_answer
    If Z is a complex number satisfying \[\left| {{Z}^{3}}+{{Z}^{-3}} \right|\le 2,\] then the maximum possible value of \[\left| Z+{{Z}^{-\,1}} \right|\]is

    A) \[2\]

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

    C) \[2\sqrt{2}\]

    D) \[1\]

    Correct Answer: A

    Solution :

    Given, \[\left| {{Z}^{3}}+\frac{1}{{{Z}^{3}}} \right|\le 2\]
    \[\left| {{Z}^{3}}+\frac{1}{{{Z}^{3}}} \right|\le \left| {{Z}^{3}} \right|+\frac{1}{\left| {{Z}^{3}} \right|}\le 2\]
    By        \[AM\ge GM\]
    \[\therefore \]      \[\left| {{Z}^{3}} \right|+\frac{1}{\left| {{Z}^{3}} \right|}=2\,\,\left| Z \right|=1\]
    \[\therefore \]      \[\left| Z+\frac{1}{Z} \right|\le \left| Z \right|+\frac{1}{\left| Z \right|}=2\]
    \[\therefore \]      Maximum value of \[\left| Z+\frac{1}{Z} \right|=2\]


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