JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Types of matrices, Algebra of matrices

  • question_answer
    The identity element in the group \[M=\left\{ \left. \left( \begin{matrix}    x & x  \\    x & x  \\ \end{matrix} \right) \right|x\in R;\,x\ne 0\, \right\}\] with respect to matrix multiplication is [Karnataka CET 2005]

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

    B) \[\frac{1}{2}\left( \begin{matrix}    1 & 1  \\    1 & 1  \\ \end{matrix} \right)\]

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

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

    Correct Answer: B

    Solution :

      Let \[\left[ \begin{matrix}    a & a  \\    a & a  \\ \end{matrix} \right]\]be the identity element then \[\left[ \begin{matrix}    x & x  \\    x & x  \\ \end{matrix} \right]\left[ \begin{matrix}    a & a  \\    a & a  \\ \end{matrix} \right]=\left[ \begin{matrix}    x & x  \\    x & x  \\ \end{matrix} \right]\] i.e., \[2ax=x\Rightarrow a=\frac{1}{2}\], \[(\because x\ne 0)\] Identity element =\[\frac{1}{2}\left[ \begin{matrix}    1 & 1  \\    1 & 1  \\ \end{matrix} \right]\].


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