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

  • question_answer
    If \[A=\left[ \begin{matrix}    0 & 1 & 3  \\    1 & 2 & 3  \\    3 & a & 1  \\ \end{matrix} \right]\] and \[{{A}^{-1}}=\left[ \begin{matrix}    1/2 & -1/2 & 1/2  \\    -4 & 3 & c  \\    5/2 & -3/2 & 1/2  \\ \end{matrix} \right]\] Then the value of \[a+c\]is equal to

    A)  1

    B) 0

    C) 2

    D) None of these

    Correct Answer: B

    Solution :

    [b] We have, \[I=A{{A}^{-1}}\] \[=\frac{1}{2}\left[ \begin{matrix}    0 & 1 & 2  \\    1 & 2 & 3  \\    3 & a & 1  \\ \end{matrix} \right]\left[ \begin{matrix}    1 & -1 & 1  \\    -8 & 6 & 2c  \\    5 & -3 & 1  \\ \end{matrix} \right]\] \[=\left[ \begin{matrix}    1 & 0 & c+1  \\    0 & 1 & 2(c+1)  \\    4(1-a) & 3(a-1) & 2+ac  \\ \end{matrix} \right]=\left[ \begin{matrix}    1 & 0 & 0  \\    0 & 1 & 0  \\    0 & 0 & 1  \\ \end{matrix} \right]\] Comparing the elements we get \[c+1=0\Rightarrow c=-1\] and \[a-1=0\Rightarrow a=1\]


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