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

  • question_answer
    If \[A=\left[ \begin{matrix}    2 & 2  \\    a & b  \\ \end{matrix} \right]\]and  \[{{A}^{2}}=O\], then \[(a,b)=\]

    A) \[(-2,\,-2)\]

    B) \[(2,\,-2)\]

    C) \[(-2,\,2)\]

    D) \[(2,\,2)\]

    Correct Answer: A

    Solution :

      \[{{A}^{2}}=\left[ \begin{matrix}    2 & 2  \\    a & b  \\ \end{matrix} \right]\,\left[ \begin{matrix}    2 & 2  \\    a & b  \\ \end{matrix} \right]=\left[ \begin{matrix}    4+2a & 4+2b  \\    2a+ab & 2a+{{b}^{2}}  \\ \end{matrix} \right]=0=\left[ \begin{matrix}    0 & 0  \\    0 & 0  \\ \end{matrix} \right]\] \[\Rightarrow \,\,4+2a=0,4+2b=0,\]\[2a+ab=0,\] \[2a+{{b}^{2}}=0\]must be consistent. \[\Rightarrow \] \[a=-2\], \[b=-2\].


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