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

  • question_answer
    \[{{(27)}^{1/3}}=\]

    A) 3

    B) \[3,\,\,3i,\,3{{i}^{2}}\]

    C) \[3,\,3\omega ,\,3{{\omega }^{2}}\]

    D) None of these

    Correct Answer: C

    Solution :

    Let  \[x={{27}^{1/3}}\] \[={{x}^{3}}-27=0\] Þ \[(x-3)({{x}^{2}}+3x+9)=0\] \[x=3,x=3\left( \frac{-1\pm i\sqrt{3}}{2} \right)\].  Hence, roots are\[3,\,3\omega ,3{{\omega }^{2}}\]. Trick: As we know \[{{(27)}^{1/3}}\] must have 3 roots, so (a) option cannot be the best. Here (c) satisfies.


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