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
    A root of the equation \[\left| \,\begin{matrix}    3-x & -6 & 3  \\    -6 & 3-x & 3  \\    3 & 3 & -6-x  \\ \end{matrix}\, \right|=0\]is  [Roorkee 1991; RPET 2001; J & K 2005]

    A) 6

    B) 3

    C) 0

    D) None of these

    Correct Answer: C

    Solution :

    Applying\[{{C}_{1}}\to {{C}_{1}}+{{C}_{2}}+{{C}_{3}}\], we obtain \[-x\,\left| \,\begin{matrix}    1 & -6 & 3  \\    1 & 3-x & 3  \\    1 & 3 & -6-x  \\ \end{matrix}\, \right|\] = 0 \[\Rightarrow \]\[-x\,\left| \,\begin{matrix}    1 & -6 & 3  \\    0 & 9-x & 0  \\    0 & 9 & -9-x  \\ \end{matrix}\, \right|=0\] \[\Rightarrow \] \[-x(9-x)\,(-9-x)=0\Rightarrow x=0,\,9,\,-9\]. Trick: Check by assuming the values of x from the given options.


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