JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Expansion of determinants, Solution of equation in the form of determinants and properties of determinants

  • question_answer
    If \[{{a}^{2}}+{{b}^{2}}+{{c}^{2}}=-2\]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 [AIEEE 2005]

    A) 3

    B) 2

    C) 1

    D) 0

    Correct Answer: B

    Solution :

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


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