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

  • question_answer
    If \[{{A}^{k}}=0\] (A is nilpotent with index k),\[{{(I-A)}^{p}}=I+A+{{A}^{2}}+....+{{A}^{k-1}},\] thus p is,

    A) -1

    B) -2

    C) ½

    D) None of these

    Correct Answer: A

    Solution :

    [a] Let \[B=I+A+{{A}^{2}}+....+{{A}^{k-1}}\] Now multiply both sides by (I - A), we get \[B(I-A)=(I+A+{{A}^{2}}+....+{{A}^{k-1}}(I-A)\] \[=I-A+A-{{A}^{2}}+{{A}^{2}}-{{A}^{3}}+....-{{A}^{k-1}}+{{A}^{k-1}}-{{A}^{k}}\] \[=I-{{A}^{k}}=I,\] since \[{{A}^{k}}=0\Rightarrow B={{(I-A)}^{-1}}\] Hence \[{{(I-A)}^{-1}}=I+A+{{A}^{2}}+...+{{A}^{k-1}}\] Thus \[p=-1\]


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