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

  • question_answer
    If  \[\alpha ,\beta ,\gamma \] are the cube roots of  \[p(p<0)\], then for any \[x,y\] and \[z,\,\,\frac{x\alpha +y\beta +z\gamma }{x\beta +y\gamma +z\alpha }=\] [IIT 1989]

    A) \[\frac{1}{2}(-1+i\sqrt{3})\]

    B) \[\frac{1}{2}(1+i\sqrt{3})\]

    C) \[\frac{1}{2}(1-i\sqrt{3})\]

    D) None of these

    Correct Answer: A

    Solution :

    Since\[p<0\]. Let\[p=-q\], where \[q\]is positive. Therefore \[{{p}^{1/3}}=-{{q}^{1/3}}{{(1)}^{1/3}}.\] Hence\[\alpha =-{{q}^{1/3}}\], \[\beta =-{{q}^{1/3}}\omega \]and \[\gamma =-{{q}^{1/3}}{{\omega }^{2}}\] The given expression \[\frac{x+y\omega +z{{\omega }^{2}}}{x\omega +y{{\omega }^{2}}+z}=\frac{1}{\omega }.\frac{z\omega +y{{\omega }^{2}}+z}{x\omega +y{{\omega }^{2}}+z}\]\[=\frac{1}{\omega }={{\omega }^{2}}\].


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