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

  • question_answer
    If \[\omega \] is a cube root of unity but not equal to 1 then minimum value of \[|a+b\omega +c{{\omega }^{2}}|\] (where a, b, c are integers but not all equal) is [IIT Screening 2005]

    A) 0

    B) \[\frac{\sqrt{3}}{2}\]

    C) 1

    D) 2

    Correct Answer: C

    Solution :

    Let \[y=|a+b\omega +c{{\omega }^{2}}|\] for y to be minimum \[{{y}^{2}}\] must be minimum. \[{{y}^{2}}=|a+b\omega +c{{\omega }^{2}}{{|}^{2}}\] \[{{y}^{2}}=(a+b\omega +c{{\omega }^{2}})\,(a+b\omega +c{{\omega }^{2}})\]     = \[\frac{1}{2}[{{(a-b)}^{2}}+{{(b-c)}^{2}}+{{(c-a)}^{2}}]\] Since a, b and c are not equal at a time so minimum value of \[{{y}^{2}}\] occurs when any two are same and third is differ by 1. Þ Minimum of \[y=1\] (as \[a,b,c\] are integers)


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