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

  • question_answer
    If \[A=\left[ \begin{matrix}    \alpha  & \beta   \\    \gamma  & \delta   \\ \end{matrix} \right]\] such that \[{{A}^{2}}\] is a two - rowed unit matrix, then \[\delta \] is equal to

    A) \[\alpha \]

    B) \[\beta \]

    C) \[\gamma \]

    D) None of these

    Correct Answer: A

    Solution :

    [a] We have, \[\left[ \begin{matrix}    1 & 0  \\    0 & 1  \\ \end{matrix} \right]=\left[ \begin{matrix}    \alpha  & \beta   \\    \gamma  & \delta   \\ \end{matrix} \right]\left[ \begin{matrix}    \alpha  & \beta   \\    \gamma  & \delta   \\ \end{matrix} \right]\] \[=\left[ \begin{matrix}    {{\alpha }^{2}}+\beta \gamma  & \alpha \beta +\beta \delta   \\    \alpha \gamma +\delta \gamma  & \beta \gamma +{{\delta }^{2}}  \\ \end{matrix} \right]\] \[\Rightarrow {{\alpha }^{2}}+\beta \gamma =1,\beta (\alpha +\delta )=0,\] \[\gamma (\alpha +\delta )=0,\beta \gamma +{{\delta }^{2}}=1\] \[\Rightarrow \beta =0=\gamma ,\alpha \ne -\delta \] and \[{{\alpha }^{2}}={{\delta }^{2}}\Rightarrow \delta =\alpha \]


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