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

  • question_answer
    . Which of the following is a fourth root of \[\frac{1}{2}+\frac{i\sqrt{3}}{2}\] [Karnataka CET 2003]

    A) \[cis\left( \frac{\pi }{2} \right)\]

    B) \[cis\left( \frac{\pi }{12} \right)\]

    C) \[cis\left( \frac{\pi }{6} \right)\]

    D) \[cis\left( \frac{\pi }{3} \right)\]

    Correct Answer: B

    Solution :

    \[\frac{1}{2}+i\frac{\sqrt{3}}{2}\]\[=\left( \cos \frac{\pi }{3}+i\sin \frac{\pi }{3} \right)\] Now \[{{\left( \frac{1}{2}+i\frac{\sqrt{3}}{2} \right)}^{1/4}}={{\left( \cos \frac{\pi }{3}+i\sin \frac{\pi }{3} \right)}^{1/4}}\]                                      \[=\left( \cos \frac{\pi }{12}+i\sin \frac{\pi }{12} \right)\]\[=cis\,\left( \frac{\pi }{12} \right)\].


You need to login to perform this action.
You will be redirected in 3 sec spinner