JEE Main & Advanced JEE Main Paper (Held On 09-Jan-2019 Morning)

  • question_answer
    If \[A\,\,=\,\,\left[ \begin{align}  & \cos \,\theta \,\,\,\,\,\,-\sin \theta  \\  & \sin \,\theta \,\,\,\,\,\,\,\,\,\,\,\cos \,\theta  \\ \end{align} \right]\] then matrix \[{{A}^{-50}}\] when \[\theta =\frac{\pi }{12}\], is equal to: [JEE Main Online Paper (Held On 09-Jan-2019 Morning]

    A) \[\left| \begin{align}   & \frac{1}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{\sqrt{3}}{2} \\  & -\frac{\sqrt{3}}{2}\,\,\,\,\,\,\,\,\frac{1}{2} \\ \end{align} \right|\]

    B) \[\left| \begin{align}   & \frac{1}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,-\,\frac{\sqrt{3}}{2} \\  & \frac{\sqrt{3}}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{1}{2} \\ \end{align} \right|\]

    C) \[\left| \begin{align}   & \frac{\sqrt{3}}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{1}{2}\,\,\, \\  & -\frac{1}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{\sqrt{3}}{2} \\ \end{align} \right|\]

    D) \[\left| \begin{align}   & \frac{\sqrt{3}}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,-\frac{1}{2}\,\,\, \\  & \frac{1}{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{\sqrt{3}}{2} \\ \end{align} \right|\]

    Correct Answer: C

    Solution :

    \[A\,\,=\,\,\left[ \begin{align}   & \cos \,\theta \,\,\,\,-\,\sin \,\theta  \\  & \sin \,\theta \,\,\,\,\,\,\,\,\cos \,\theta  \\ \end{align} \right]\] \[{{A}^{50}}\,\,=\,\,\left[ \begin{align}   & \cos \,50\,\theta \,\,\,\,\,\,\,-\,\sin \,50\,\theta  \\  & -\sin \,50\,\theta \,\,\,\,\,\,\,\cos \,50\,\theta  \\ \end{align} \right]\] \[{{({{A}^{-1}})}^{50}}\,\,=\,\,\left[ \begin{align}   & \cos \,50\,\theta \,\,\,\,\,\,\,\,\,\,\sin \,50\,\theta  \\  & -\sin \,50\,\theta \,\,\,\,\,\,\,\cos \,50\,\theta  \\ \end{align} \right]\] At \[\theta \,\,=\,\,\frac{\pi }{12}\] \[{{({{A}^{-1}})}^{50}}\,\,=\,\,\left[ \begin{align}   & \cos \,\frac{50\,\pi }{12}\,\,\,\,\,\,\,\,\,\,\sin \,\frac{50\,\pi }{12} \\  & -\sin \,\frac{50\,\pi }{12}\,\,\,\,\,\,\,\cos \,\frac{50\,\pi }{12}\, \\ \end{align} \right]\] \[=\,\,\left[ \begin{align}   & \frac{\sqrt{3}}{2}\,\,\,\,\,\,\,\,\,\,\frac{1}{2} \\  & -\frac{1\,}{2}\,\,\,\,\,\,\,\,\frac{\sqrt{3}}{2}\, \\ \end{align} \right]\]


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