JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Special types of matrices, Transpose, Adjoint and Inverse of matrices

  • question_answer
    \[A=\left[ \begin{matrix}    1 & 0 & 0  \\    0 & 1 & 1  \\    0 & -2 & 4  \\ \end{matrix} \right];\,\,I=\left[ \begin{matrix}    1 & 0 & 0  \\    0 & 1 & 0  \\    0 & 0 & 1  \\ \end{matrix} \right]\]\[{{A}^{-1}}=\frac{1}{6}[{{A}^{2}}+cA+dI]\] where \[c,d\in R\], then pair of values \[(c,d)\] [IIT Screening 2005]

    A) (6, 11)

    B) (6, -11)

    C) (-6, 11)

    D) (-6, -11)

    Correct Answer: C

    Solution :

    Given \[A=\left[ \begin{matrix}    1 & 0 & 0  \\    0 & 1 & 1  \\    0 & -2 & 4  \\ \end{matrix} \right],\,\,{{A}^{-1}}=\frac{1}{6}\left[ \begin{matrix}    6 & 0 & 0  \\    0 & 4 & -1  \\    0 & 2 & 1  \\ \end{matrix} \right]\] \[{{A}^{2}}=\left[ \begin{matrix}    1 & 0 & 0  \\    0 & 1 & 1  \\    0 & -2 & 4  \\ \end{matrix} \right]\,\left[ \begin{matrix}    1 & 0 & 0  \\    0 & 1 & 1  \\    0 & -2 & 4  \\ \end{matrix} \right]\,=\left[ \begin{matrix}    1 & 0 & 0  \\    0 & -1 & 5  \\    0 & -10 & 14  \\ \end{matrix} \right]\] \[cA=\left[ \begin{matrix}    c & 0 & 0  \\    0 & c & c  \\    0 & -2c & 4c  \\ \end{matrix} \right]\,\]; \[dI=\left[ \begin{matrix}    d & 0 & 0  \\    0 & d & 0  \\    0 & 0 & d  \\ \end{matrix} \right]\] \[\therefore \]By \[{{A}^{-1}}=\frac{1}{6}[{{A}^{2}}+cA+dI]\] \[\Rightarrow 6=1+c+d\],(By equality  of matrices) \ (-6, 11) satisfy the relation.


You need to login to perform this action.
You will be redirected in 3 sec spinner