JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Self Evaluation Test - Matrices

  • question_answer
    If \[A=\left[ \begin{matrix}    \cos \theta  & \sin \theta   \\    -\sin \theta  & \cos \theta   \\ \end{matrix} \right]\] then \[\underset{n\to \infty }{\mathop{\lim }}\,\frac{1}{n}{{A}^{n}}\] is

    A) A null matrix

    B) An identity matrix

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

    D) None of these

    Correct Answer: A

    Solution :

    [a] \[{{A}^{n}}=\left[ \begin{matrix}    \cos n\theta  & \sin n\theta   \\    -\sin n\theta  & \cos n\theta   \\ \end{matrix} \right]\] \[\frac{1}{n}{{A}^{n}}=\left[ \begin{matrix}    \frac{\cos n\theta }{n} & \frac{\sin n\theta }{n}  \\    -\frac{\sin n\theta }{n} & \frac{\cos n\theta }{n}  \\ \end{matrix} \right]\] But \[-1\le \cos n\theta \le 1\] and \[-1\le sinn\theta \le 1\] \[\underset{n\to \infty }{\mathop{\lim }}\,\frac{\sin n\theta }{n}=0,\underset{n\to \infty }{\mathop{\lim }}\,\frac{\cos n\theta }{n}=0\] \[\underset{n\to \infty }{\mathop{\lim }}\,\frac{1}{n}{{A}^{n}}=\left[ \begin{matrix}    0 & 0  \\    0 & 0  \\ \end{matrix} \right]\]


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