JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Types of matrices, Algebra of matrices

  • question_answer
    If \[A=\left[ \begin{matrix}    \lambda  & 1  \\    -1 & -\lambda   \\ \end{matrix} \right]\], then for what value of \[\lambda ,\,{{A}^{2}}=O\]  [MP PET 1992]

    A) 0

    B) \[\pm \text{ }1\]

    C) - 1

    D) 1

    Correct Answer: B

    Solution :

     \[{{A}^{2}}=A\,.\,A=\left[ \begin{matrix}    \lambda  & 1  \\    -1 & -\lambda   \\ \end{matrix} \right]\,\left[ \begin{matrix}    \lambda  & 1  \\    -1 & -\lambda   \\ \end{matrix} \right]=\left[ \begin{matrix}    {{\lambda }^{2}}-1 & 0  \\    0 & -1+{{\lambda }^{2}}  \\ \end{matrix} \right]=0\](As given) Þ \[{{\lambda }^{2}}-1=0\Rightarrow \lambda =\pm 1\].


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