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 \[\left| \,\begin{matrix}    -{{a}^{2}} & ab & ac  \\    ab & -{{b}^{2}} & bc  \\    ac & bc & -{{c}^{2}}  \\ \end{matrix}\, \right|=K{{a}^{2}}{{b}^{2}}{{c}^{2}},\]then \[K=\] [Kurukshetra CEE 1996, 98, 2002; RPET 1997; MP PET 1998, 99; Tamilnadu (Engg.) 2002]

    A) - 4

    B) 2

    C) 4

    D) 8

    Correct Answer: C

    Solution :

    \[\left| \,\begin{matrix}    -{{a}^{2}} & ab & ac  \\    ab & -{{b}^{2}} & bc  \\    ac & bc & -{{c}^{2}}  \\ \end{matrix} \right|=abc\left| \,\begin{matrix}    -a & b & c  \\    a & -b & c  \\    a & b & -c  \\ \end{matrix}\, \right|\] \[=(abc)(abc)\left| \,\begin{matrix}    -1 & 1 & 1  \\    1 & -1 & 1  \\    1 & 1 & -1  \\ \end{matrix} \right|={{a}^{2}}{{b}^{2}}{{c}^{2}}(-1)(-4)\] \[=4{{a}^{2}}{{b}^{2}}{{c}^{2}}=K{{a}^{2}}{{b}^{2}}{{c}^{2}}\], (given) Þ K = 4.


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