JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank De Moivre's theorem and Roots of unity

  • question_answer
    Let \[x=\alpha +\beta ,\,y=\alpha \omega +\beta {{\omega }^{2}},\,z=\alpha {{\omega }^{2}}+\beta \omega ,\,\omega \] is an imaginary cube root of unity. Product of xyz is  [Orissa JEE 2005]

    A) \[{{\alpha }^{2}}+{{\beta }^{2}}\]

    B) \[{{\alpha }^{2}}-{{\beta }^{2}}\]

    C) \[{{\alpha }^{3}}+{{\beta }^{3}}\]

    D) \[{{\alpha }^{3}}-{{\beta }^{3}}\]

    Correct Answer: D

    Solution :

    \[x=\alpha +\beta ,\,y=\alpha \omega +\beta {{\omega }^{2}},\,z=\alpha {{\omega }^{2}}+\beta \omega \] \[\therefore \] \[xyz=(\alpha +\beta )\,(\alpha \omega +\beta {{\omega }^{2}})(\alpha {{\omega }^{2}}+\beta \omega )\]             = \[(\alpha +\beta )\,[{{\alpha }^{2}}+\alpha \beta (\omega +{{\omega }^{2}})+{{\beta }^{2}}]\]             = \[(\alpha +\beta )\,({{\alpha }^{2}}-\alpha \beta +{{\beta }^{2}})={{\alpha }^{3}}+{{\beta }^{3}}\].


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