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

  • question_answer
    If \[A=\left[ \begin{matrix}    4 & 1  \\    3 & 2  \\ \end{matrix} \right]\]and \[I=\left[ \begin{matrix}    1 & 0  \\    0 & 1  \\ \end{matrix} \right]\], \[{{A}^{2}}-6A=\] [MP PET 1987]

    A) 3I

    B) 5I

    C) - 5I

    D) None of these

    Correct Answer: C

    Solution :

      \[{{A}^{2}}-6\,A=\left[ \begin{matrix}    4 & 1  \\    3 & 2  \\ \end{matrix} \right]\,\left[ \begin{matrix}    4 & 1  \\    3 & 2  \\ \end{matrix} \right]-6\left[ \begin{matrix}    4 & 1  \\    3 & 2  \\ \end{matrix} \right]\]  \[=\left[ \begin{matrix}    19 & 6  \\    18 & 7  \\ \end{matrix} \right]-\left[ \begin{matrix}    24 & 6  \\    18 & 12  \\ \end{matrix} \right]\]\[\left[ \begin{matrix}    -5 & 0  \\    0 & -5  \\ \end{matrix} \right]=-5I\].


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