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

  • question_answer
    If \[\pi /3\] is a complex root of the equation \[{{z}^{3}}=1\], then \[\omega +{{\omega }^{\left( \frac{1}{2}\,+\,\frac{3}{8}\,+\,\frac{9}{32}\,+\,\frac{27}{128}\,+... \right)}}\] is equal to [Roorkee 2000; AMU 2005]

    A) - 1

    B) 0

    C) 9

    D) i

    Correct Answer: A

    Solution :

    \[\omega +{{\omega }^{\left( \frac{1}{2}+\frac{3}{8}+\frac{9}{32}+\frac{27}{128}+..... \right)}}\] \[=\frac{(1-\cos \theta )-i\sin \theta }{{{(1-\cos \theta )}^{2}}+{{\sin }^{2}}\theta }\] \[\omega +{{\omega }^{\left( \frac{1/2}{1-3/4} \right)}}\] \[\Rightarrow \] \[\omega +{{\omega }^{2}}=-1\]     \[[\because 1+\omega +{{\omega }^{2}}=0]\]


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