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

  • question_answer
    If \[A=\left[ \begin{matrix}    \alpha  & 0  \\    1 & 1  \\ \end{matrix} \right]\] and \[B=\left[ \begin{matrix}    9 & a  \\    b & c  \\ \end{matrix} \right]\] and \[{{A}^{2}}=B\], then the value of a + b + c is

    A) 1 or -1

    B) 5 or -1

    C) 5 or 1

    D) no real values

    Correct Answer: B

    Solution :

    [b] We have \[{{A}^{2}}=\left[ \begin{matrix}    \alpha  & 0  \\    1 & 1  \\ \end{matrix} \right]\left[ \begin{matrix}    \alpha  & 0  \\    1 & 1  \\ \end{matrix} \right]=\left[ \begin{matrix}    {{\alpha }^{2}} & 0  \\    \alpha +1 & 1  \\ \end{matrix} \right]=\left[ \begin{matrix}    9 & a  \\    b & c  \\ \end{matrix} \right]\] \[\Rightarrow \] we get \[{{\alpha }^{2}}=9\Rightarrow \alpha =\pm 3\] and \[a=0,c=1,b=\alpha +1=3+1=4\] or \[b=-3+1=-2\] So \[a+b+c=(0+4+1)=5\] or \[(0-2+1)=-1\]


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