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

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    If \[A=\left[ \begin{matrix}    \cos x & \sin x  \\    -\sin x & \cos x  \\ \end{matrix} \right]\], then \[A.\]\[(adj(A))\]= [RPET  2003]

    A) \[\left[ \begin{matrix}    1 & 0  \\    0 & 1  \\ \end{matrix} \right]\]

    B) \[\left[ \begin{matrix}    0 & 1  \\    1 & 0  \\ \end{matrix} \right]\]

    C) \[\left[ \begin{matrix}    1 & 1  \\    0 & 0  \\ \end{matrix} \right]\]

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

    Correct Answer: A

    Solution :

    \[A.(adj\,(A)=|A|\,I\] Here\[|A|\,={{\cos }^{2}}x+{{\sin }^{2}}x=1\]. Hence, \[A.(adj\,(A))\,=\,\left[ \begin{matrix}    1 & 0  \\    0 & 1  \\ \end{matrix} \right]\].


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