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

  • question_answer
    The common roots of the equations \[{{x}^{12}}-1=0\], \[{{x}^{4}}+{{x}^{2}}+1=0\] are [EAMCET 1989]

    A) \[\pm \omega \]

    B) \[\pm {{\omega }^{2}}\]

    C) \[\pm \omega ,\,\pm {{\omega }^{2}}\]

    D) None of these

    Correct Answer: C

    Solution :

      \[{{x}^{12}}-1=({{x}^{6}}+1)({{x}^{6}}-1)=({{x}^{6}}+1)({{x}^{2}}-1)({{x}^{4}}+{{x}^{2}}+1)\] Common roots are given by \[{{x}^{4}}+{{x}^{2}}+1=0\] \[\therefore \,\,\,\]\[{{x}^{2}}=\frac{-1\pm i\sqrt{3}}{2}=\omega ,{{\omega }^{2}}\]or \[{{\omega }^{4}},{{\omega }^{2}}\]      \[(\because {{\omega }^{3}}=1)\] or \[x=\pm {{\omega }^{2}},\pm \omega \]


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