CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2005

  • question_answer
    If \[\omega \] is a complex cube-root of unity, then\[\left| \begin{matrix}    1 & \omega  & {{\omega }^{2}}  \\    \omega  & {{\omega }^{2}} & 1  \\    {{\omega }^{2}} & 1 & \omega   \\ \end{matrix} \right|\] is equal to :

    A)  -1                                          

    B)  1

    C)  0

    D)  \[\omega \]

    Correct Answer: C

    Solution :

    Let \[A=\left| \begin{matrix}    1 & \omega  & {{\omega }^{2}}  \\    \omega  & {{\omega }^{2}} & 1  \\    {{\omega }^{2}} & 1 & \omega   \\ \end{matrix} \right|\]                 \[=\left| \begin{matrix}    1+\omega +{{\omega }^{2}} & \omega  & {{\omega }^{2}}  \\    1+\omega +{{\omega }^{2}} & {{\omega }^{2}} & 1  \\    1+\omega +{{\omega }^{2}} & 1 & \omega   \\ \end{matrix} \right|\]                                                 \[{{C}_{1}}\to {{C}_{1}}+{{C}_{2}}+{{C}_{3}}\]                 \[=\left| \begin{matrix}    0 & \omega  & {{\omega }^{2}}  \\    0 & {{\omega }^{2}} & 1  \\    0 & 1 & \omega   \\ \end{matrix} \right|\]          \[[\because \,\,1+\omega +{{\omega }^{2}}=0]\]                 = 0


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