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

  • question_answer
    If \[{{z}_{1}},{{z}_{2}},{{z}_{3}}......{{n}_{n}}\] are nth, roots of unity, then for \[k=1,\,2,.....,n\]

    A) \[|{{z}_{k}}|=k|{{z}_{k+1}}|\]

    B) \[|{{z}_{k+1}}|=k|{{z}_{k}}|\]

    C) \[|{{z}_{k+1}}|\,=\,|{{z}_{k}}|+|{{z}_{k+1}}|\]

    D) \[|{{z}_{k}}|=|{{z}_{k+1}}|\]

    Correct Answer: D

    Solution :

    The \[{{n}^{\text{th}}}\]roots of unity are given by \[{{z}_{k}}={{e}^{\frac{i2\pi (k-1)}{n}}},\,\,\,\,\,(k=1,2....,n)\] \ \[|{{z}_{k}}|\,=\,\left| \,{{e}^{\frac{i2\pi (k-1)}{n}}}\, \right|=1\]for all \[k=1,2,.....,n\] Þ \[|{{z}_{k}}|\,=\,|{{z}_{k+1}}|\]for all \[k=1,2.....,n\]


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