JEE Main & Advanced JEE Main Paper (Held On 12-Jan-2019 Morning)

  • question_answer
    Let \[P=\left[ \begin{matrix}    1 & 0 & 0  \\    3 & 1 & 0  \\    9 & 3 & 1  \\ \end{matrix} \right]\]and \[Q=[{{q}_{ij}}]\]be two \[3\times 3\]matrices such that \[Q-{{P}^{5}}={{I}_{3}}.\]Then \[\frac{{{q}_{21}}+{{q}_{31}}}{{{q}_{32}}}\]is equal to             [JEE Main Online Paper Held On 12-Jan-2019 Morning]              

    A) 10                                

    B) 135  

    C) 9                                 

    D)   15

    Correct Answer: A

    Solution :

    Here, \[P=\left[ \begin{matrix}    1 & 0 & 0  \\    3 & 1 & 0  \\    9 & 3 & 1  \\ \end{matrix} \right]\]and\[Q={{P}^{5}}+{{I}_{3}}\] \[\therefore \]\[{{P}^{2}}=\left[ \begin{matrix}    1 & 0 & 0  \\    3 & 1 & 0  \\    9 & 3 & 1  \\ \end{matrix} \right]\left[ \begin{matrix}    1 & 0 & 0  \\    3 & 1 & 0  \\    9 & 3 & 1  \\ \end{matrix} \right]=\left[ \begin{matrix}    1 & 0 & 0  \\    6 & 1 & 0  \\    27 & 6 & 1  \\ \end{matrix} \right]\] \[{{P}^{3}}=\left[ \begin{matrix}    1 & 0 & 0  \\    6 & 1 & 0  \\    27 & 6 & 1  \\ \end{matrix} \right]\left[ \begin{matrix}    1 & 0 & 0  \\    3 & 1 & 0  \\    9 & 3 & 1  \\ \end{matrix} \right]=\left[ \begin{matrix}    1 & 0 & 0  \\    9 & 1 & 0  \\    54 & 9 & 1  \\ \end{matrix} \right]\] Similarly, \[{{P}^{5}}=\left[ \begin{matrix}    1 & 0 & 0  \\    15 & 1 & 0  \\    135 & 15 & 1  \\ \end{matrix} \right]\] \[\therefore \]\[Q=\left[ \begin{matrix}    2 & 0 & 0  \\    15 & 2 & 0  \\    135 & 15 & 2  \\ \end{matrix} \right]\] \[\Rightarrow \]\[\frac{{{q}_{21}}+{{q}_{31}}}{{{q}_{32}}}=\frac{15+135}{15}=10\]


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