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

  • question_answer
    If \[\left| \begin{matrix}    {{x}^{n}} & {{x}^{n+2}} & {{x}^{2n}}  \\    1 & {{x}^{a}} & a  \\    {{x}^{n+5}} & {{x}^{a+6}} & {{x}^{2n+5}}  \\ \end{matrix} \right|=0\,\forall \,x\,\in R,\] where \[n\in N\] then value of 'a' is

    A) \[n\]

    B) \[n-1\]

    C) \[n+1\]

    D) None of these

    Correct Answer: C

    Solution :

    [c] Taking \[{{x}^{5}}\] common from last row, we get, \[{{x}^{5}}\left| \begin{matrix}    {{x}^{n}} & {{x}^{n+2}} & {{x}^{2n}}  \\    1 & {{x}^{a}} & a  \\    {{x}^{n}} & {{x}^{a+1}} & {{x}^{2n}}  \\ \end{matrix} \right|=0\,\,\,\,x\in R,\] \[\Rightarrow a+1=n+2\Rightarrow a=n+1\] (as it will make first and third row identical)


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