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

  • question_answer
    \[(1-\omega +{{\omega }^{2}})(1-{{\omega }^{2}}+{{\omega }^{4}})(1-{{\omega }^{4}}+{{\omega }^{8}})...........\]to \[2n\] factors is [EAMCET 1988]

    A) \[{{2}^{n}}\]

    B) \[{{2}^{2n}}\]

    C) 0

    D) 1

    Correct Answer: B

    Solution :

    \[{{\omega }^{4}}=\omega ,{{\omega }^{8}}={{\omega }^{2}}\]etc. 3rd, 5th, 7th factors are each equal to 1st and 4th, 6th, 8th factors are each equal to 2nd. L.H.S \[=(-2\omega )(-2{{\omega }^{2}})(-2\omega )(-2{{\omega }^{2}}).....\]to \[2n\] factors                 \[=({{2}^{2}}{{\omega }^{3}})({{2}^{2}}{{\omega }^{3}})......\]to\[n\]factors \[={{({{2}^{2}})}^{n}}={{2}^{2n}}\]


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