JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Self Evaluation Test - Determinats

  • question_answer
    If \[A=\left[ \begin{matrix}    a & 0 & 0  \\    0 & a & 0  \\    0 & 0 & a  \\ \end{matrix} \right],\] then the value of \[|adj\,\,A|\] is

    A) \[{{a}^{27}}\]

    B) \[{{a}^{9}}\]

    C) \[{{a}^{6}}\]

    D) \[{{a}^{2}}\]

    Correct Answer: C

    Solution :

    [c] \[Cofactor\,\,matrix=\left[ \begin{matrix}    {{a}^{2}} & 0 & 0  \\    0 & {{a}^{2}} & 0  \\    0 & 0 & {{a}^{2}}  \\ \end{matrix} \right]\] \[\therefore \] adj A = (cofactor matrix)\[=\left[ \begin{matrix}    {{a}^{2}} & 0 & 0  \\    0 & {{a}^{2}} & 0  \\    0 & 0 & {{a}^{2}}  \\ \end{matrix} \right]\] \[\therefore \left| adj\,\,A \right|=\left[ \begin{matrix}    {{a}^{2}} & 0 & 0  \\    0 & {{a}^{2}} & 0  \\    0 & 0 & {{a}^{2}}  \\ \end{matrix} \right]={{a}^{6}}\]


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