JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Mock Test - Matrices

  • question_answer
    If \[A=\left[ \begin{matrix}    a & b  \\    b & a  \\ \end{matrix} \right]\]and \[{{A}^{2}}=\left[ \begin{matrix}    \alpha  & \beta   \\    \beta  & \alpha   \\ \end{matrix} \right]\], then

    A) \[\alpha ={{a}^{2}}+{{b}^{2}},\beta =ab\]

    B) \[\alpha ={{a}^{2}}+{{b}^{2}},\beta =2ab\]

    C) \[\alpha ={{a}^{2}}+{{b}^{2}},\beta ={{a}^{2}}-{{b}^{2}}\]

    D) \[\alpha =2ab,\,\,\beta ={{a}^{2}}+{{b}^{2}}\]

    Correct Answer: B

    Solution :

    [b] \[{{A}^{2}}=\left[ \begin{matrix}    \alpha  & \beta   \\    \beta  & \alpha   \\ \end{matrix} \right]=\left[ \begin{matrix}    a & b  \\    b & a  \\ \end{matrix} \right]\left[ \begin{matrix}    a & b  \\    b & a  \\ \end{matrix} \right]\] \[=\left[ \begin{matrix}    {{a}^{2}}+{{b}^{2}} & 2ab  \\    2ab & {{a}^{2}}+{{b}^{2}}  \\ \end{matrix} \right]\] \[\therefore \alpha ={{a}^{2}}+{{b}^{2}},\beta =2ab\]


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