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

  • question_answer
    If \[A=\left[ \begin{matrix}    1 & 2  \\    0 & 3  \\ \end{matrix} \right]\] is a \[2\times 2\] matrix and \[f(x)={{x}^{2}}-x+2\] is a polynomial, then what is \[f(A)\]?

    A) \[\left[ \begin{matrix}    1 & 7  \\    1 & 7  \\ \end{matrix} \right]\]

    B) \[\left[ \begin{matrix}    2 & 6  \\    0 & 8  \\ \end{matrix} \right]\]

    C) \[\left[ \begin{matrix}    2 & 6  \\    0 & 6  \\ \end{matrix} \right]\]

    D) \[\left[ \begin{matrix}    2 & 6  \\    0 & 7  \\ \end{matrix} \right]\]

    Correct Answer: B

    Solution :

    [b] Given that, \[A=\left[ \begin{matrix}    1 & 2  \\    0 & 3  \\ \end{matrix} \right]\] \[{{A}^{2}}=\left[ \begin{matrix}    1 & 2  \\    0 & 3  \\ \end{matrix} \right]\left[ \begin{matrix}    1 & 2  \\    0 & 3  \\ \end{matrix} \right]=\left[ \begin{matrix}    1 & 2+6  \\    0 & 9  \\ \end{matrix} \right]=\left[ \begin{matrix}    1 & 8  \\    0 & 9  \\ \end{matrix} \right]\] Since, \[f(x)={{x}^{2}}-x+2\] Putting A in place of x \[f(A)={{A}^{2}}-A+2I\] \[=\left[ \begin{matrix}    1 & 8  \\    0 & 9  \\ \end{matrix} \right]-\left[ \begin{matrix}    1 & 2  \\    0 & 3  \\ \end{matrix} \right]+\left[ \begin{matrix}    2 & 0  \\    0 & 2  \\ \end{matrix} \right]\] \[=\left[ \begin{matrix}    1-1+2 & 8-2+0  \\    0-0+0 & 9-3+2  \\ \end{matrix} \right]\] \[=\left[ \begin{matrix}    2 & 6  \\    0 & 8  \\ \end{matrix} \right]\]


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