JEE Main & Advanced JEE Main Paper (Held On 12 May 2012)

  • question_answer
    Let A and B be real matrices of the form \[\left[ \begin{matrix}   \alpha  & 0  \\   0 & \beta   \\ \end{matrix} \right]\]and \[\left[ \begin{matrix}    0 & \gamma   \\    \delta  & 0  \\ \end{matrix} \right],\]respectively. Statement 1: AB - BA is always an-invertible matrix. Statement 2: AB - BA is never an identity matrix.   JEE Main Online Paper (Held On 12 May 2012)

    A) Statement 1 is true. Statement 2 is false.

    B)                        Statement 1 is false, Statement 2 is true.

    C)                        Statement 1 is true. Statement 2 is true; Statement 2 is a correct explanation of Statement!.

    D)                        Statement 1 is true. Statement 2 is true, Statement 2 is not a correct explanation of Statement 1,

    Correct Answer: A

    Solution :

                    Let A and B be real matrices such that \[A=\left[ \begin{matrix}    \alpha  & 0  \\    0 & \beta   \\ \end{matrix} \right]\]and\[B=\left[ \begin{matrix}    0 & \gamma   \\    \delta  & 0  \\ \end{matrix} \right]\] Now,\[AB=\left[ \begin{matrix}    0 & \alpha \gamma   \\    \beta \delta  & 0  \\ \end{matrix} \right]\]and\[BA=\left[ \begin{matrix}    0 & \gamma \beta   \\    \delta \alpha  & 0  \\ \end{matrix} \right]\] Statement-1: \[AB-BA=\left[ \begin{matrix}    0 & \gamma \left( \alpha -\beta  \right)  \\    \delta \left( \beta -\alpha  \right) & 0  \\ \end{matrix} \right]\] \[|AB-BA|={{\left( \alpha -\beta  \right)}^{2}}\gamma \delta \ne 0\] \[\therefore \] AB - BA is always an invertible matrix. Hence, statement -1 is true. But AB - BA can be identity matrix if\[\gamma =-\delta \] or \[\delta =-\gamma \] So, statement - 2 is false.


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