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

  • question_answer
    \[{{\left( \frac{-1+i\sqrt{3}}{2} \right)}^{20}}+{{\left( \frac{-1-i\sqrt{3}}{2} \right)}^{20}}=\]

    A) \[20\sqrt{3}i\]

    B) 1

    C) \[\frac{1}{{{2}^{19}}}\]

    D) \[-1\]

    Correct Answer: D

    Solution :

    As \[\frac{-1+i\sqrt{3}}{2}=\omega \]and \[\frac{-1-i\sqrt{3}}{2}={{\omega }^{2}}\] \[\therefore \,\,\,{{(\omega )}^{20}}+{{({{\omega }^{2}})}^{20}}={{\omega }^{18}}.{{\omega }^{2}}+{{\omega }^{39}}.\omega ={{\omega }^{2}}+\omega =-1\]


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