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

  • question_answer
    If \[{{a}^{2}}+{{b}^{2}}+{{c}^{2}}\ne -2\] and\[f(x)=\left| \begin{matrix}    (1+{{a}^{2}})x & (1+{{b}^{2}})x & (1+{{c}^{2}})x  \\    (1+{{a}^{2}})x & (1+{{b}^{2}})x & (1+{{c}^{2}})x  \\    (1+{{a}^{2}})x & (1+{{b}^{2}})x & (1+{{c}^{2}})x  \\ \end{matrix} \right|\]then \[f(x)\] is a polynomial of degree

    A) \[1\]

    B) 0

    C) \[3\]

    D) \[2\]

    Correct Answer: D

    Solution :

    [d] \[f(x)=\] \[\left| \begin{matrix}    1+({{a}^{2}}+{{b}^{2}}+{{c}^{2}}+2)x & (1+{{b}^{2}})x & (1+{{c}^{2}})x  \\    1+({{a}^{2}}+{{b}^{2}}+{{c}^{2}}+2)x & 1+{{b}^{2}}x & (1+{{c}^{2}})x  \\    1+({{a}^{2}}+{{b}^{2}}+{{c}^{2}}+2)x & (1+{{b}^{2}})x & 1+{{c}^{2}}x  \\ \end{matrix} \right|\] Applying, \[{{C}_{1}}\to {{C}_{1}}+{{C}_{2}}+{{C}_{3}}\] \[=\left| \begin{matrix}    1 & (1+{{b}^{2}})x & (1+{{c}^{2}})x  \\    1 & 1+{{b}^{2}}x & (1+{{c}^{2}})x  \\    1 & (1+{{b}^{2}})x & 1+{{c}^{2}}x  \\ \end{matrix} \right|\] \[\therefore \,\,\,{{a}^{2}}+{{b}^{2}}+{{c}^{2}}+2\ne 0\] \[f(x)=\left| \begin{matrix}    0 & x-1 & 0  \\    0 & 1-x & x-1  \\    1 & (1+{{b}^{2}})x & 1+{{c}^{2}}x  \\ \end{matrix} \right|\] Applying  \[{{R}_{1}}\to {{R}_{1}}-{{R}_{2}},\] \[{{R}_{2}}\to {{R}_{2}}-{{R}_{3}}\] \[f(x)={{(x-1)}^{2}}\] Hence degree = 2.


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