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

  • question_answer
    If \[A=\left( \begin{matrix}    p & q  \\    0 & 1  \\ \end{matrix} \right)\], then \[{{A}^{8}}=\left( \begin{matrix}    {{p}^{8}} & q\left( \frac{{{p}^{8}}-1}{p-1} \right)  \\    0 & k  \\ \end{matrix} \right)\]. The value of k is

    A) 1

    B) 0   

    C) 2

    D) -1

    Correct Answer: A

    Solution :

    [a] \[{{A}^{2}}=\left( \begin{matrix}    p & q  \\    0 & 1  \\ \end{matrix} \right)\left( \begin{matrix}    p & q  \\    0 & 1  \\ \end{matrix} \right)=\left( \begin{matrix}    {{p}^{2}} & pq+q  \\    0 & 1  \\ \end{matrix} \right)\] \[=\left( \begin{matrix}    {{p}^{2}} & q(p+1)  \\    0 & 1  \\ \end{matrix} \right)\] \[{{A}^{3}}=\left( \begin{matrix}    p & q  \\    0 & 1  \\ \end{matrix} \right)\left( \begin{matrix}    {{p}^{2}} & pq+q  \\    0 & 1  \\ \end{matrix} \right)\] \[\left( \begin{matrix}    {{p}^{3}} & q({{p}^{2}}+p+1)  \\    0 & 1  \\ \end{matrix} \right)\] Similarly, \[{{A}^{4}}=\left( \begin{matrix}    {{p}^{4}} & q({{p}^{3}}+{{p}^{2}}+p+1)  \\    0 & 1  \\ \end{matrix} \right)\] and so on. \[\therefore {{A}^{8}}=\left( \begin{matrix}    {{p}^{8}} & q({{p}^{7}}+{{p}^{8}}+...+1)  \\    0 & 1  \\ \end{matrix} \right)=\left( \begin{matrix}    {{p}^{8}} & q\left( \frac{{{p}^{8}}-1}{p-1} \right)  \\    0 & 1  \\ \end{matrix} \right)\]


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