JEE Main & Advanced JEE Main Paper (Held On 09-Jan-2019 Evening)

  • question_answer
    Let \[{{z}_{0}}\] be a root of the quadratic equation, \[{{x}^{2}}+x+1=0\]. If \[z=3+6i\,{{z}_{0}}^{81}-\,3i\,{{z}_{0}}^{93}\], then arg z is equal to: [JEE Main Online Paper (Held On 09-Jan-2019 Evening]

    A) \[\frac{\pi }{6}\,\]    

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

    C)               0

    D)               \[\frac{\pi }{4}\,\]

    Correct Answer: D

    Solution :

    \[1+x+{{x}^{2}}=0\] \[x\,\,=\,\,\frac{-1\pm \,\sqrt{1-4}}{2}\,\,=\,\frac{-1\,\pm \,i\sqrt{3}}{2}\] \[{{z}_{0}}=w,\text{ }{{w}^{2}}\] Now   \[z=3+6i\,{{z}_{\text{0}}}^{\text{81}}-3i\,{{z}_{0}}^{93}\] \[z=3+6i{{w}_{{}}}^{81}-3i{{w}^{9}}^{3}\left( {{w}^{93}}={{w}^{8}}^{1}=1 \right)\] \[\Rightarrow \,\,\,\,\,\,z=3+3i\] then arg(z) \[~=ta{{n}^{-1}}\left( \frac{3}{3} \right)=ta{{n}^{-1}}\left( 1 \right)=\frac{\pi }{4}\]


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