JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Square root, Representation and Logarithm of complex numbers

  • question_answer
    If \[-1+\sqrt{-3}=r{{e}^{i\theta }},\]then \[\theta \] is equal to [RPET 1989; MP PET 1999]

    A) \[\frac{\pi }{3}\]

    B) \[-\frac{\pi }{3}\]

    C) \[\frac{2\pi }{3}\]

    D) \[-\frac{2\pi }{3}\]

    Correct Answer: C

    Solution :

    Here \[-1+\sqrt{-3}=r{{e}^{i\theta }}\]Þ \[-1+i\sqrt{3}=r{{e}^{i\theta }}\] \[=r\cos \theta +ir\sin \theta \] Equating real and imaginary parts, we get \[r\cos \theta =-1\]and \[r\sin \theta =\sqrt{3}\] Hence\[\tan \theta =-\sqrt{3}\,\,\Rightarrow \tan \theta =\tan \frac{2\pi }{3}\].Hence  \[\theta =\frac{2\pi }{3}\].


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