KVPY Sample Paper KVPY Stream-SX Model Paper-9

  • question_answer
    The  matrices  \[\left[ \begin{matrix}    \cos \theta  & -\sin \theta   \\    \sin \theta  & \cos \theta   \\ \end{matrix} \right]\] and  \[\left[ \begin{matrix}    a & 0  \\    0 & b  \\ \end{matrix} \right]\] commute under multiplication

    A) if \[a=b\]or\[\theta =n\pi ,\]where is an integer

    B) Always

    C) Never

    D) if \[a\,\cos \theta \ne b\sin \theta \]

    Correct Answer: A

    Solution :

    \[\left[ \begin{align}   & \cos \theta -\sin \theta  \\  & \sin \theta -\cos \theta  \\ \end{align} \right]\left[ \begin{align}   & a\,\,0 \\  & 0\,\,b \\ \end{align} \right]=\left[ \begin{align}   & a\,\cos \theta -b\sin \theta  \\  & a\sin \theta -b\cos \theta  \\ \end{align} \right]\]
    and
    \[\left[ \begin{align}   & a\,\,0 \\  & 0\,\,b \\ \end{align} \right]=\left[ \begin{align}   & \cos \theta -\sin \theta  \\  & \sin \theta -\cos \theta  \\ \end{align} \right]=\left[ \begin{align}   & a\,\cos \theta -a\sin \theta  \\  & b\sin \theta \,b\cos \theta  \\ \end{align} \right]\]
    \[a\,\sin \theta =b\sin \theta \Rightarrow (a-b)sin\theta =0\]
    Either a=b\[or\sin \theta =0\Rightarrow \theta =n\pi ;n\in \mathbb{Z}\]


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