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 \[a,b,c\]are unequal what is the condition that the value of the following determinant is zero \[\Delta =\left| \,\begin{matrix}    a & {{a}^{2}} & {{a}^{3}}+1  \\    b & {{b}^{2}} & {{b}^{3}}+1  \\    c & {{c}^{2}} & {{c}^{3}}+1  \\ \end{matrix}\, \right|\] [IIT 1985; DCE 1999]

    A) \[1+abc=0\]

    B) \[a+b+c+1=0\]

    C) \[(a-b)(b-c)(c-a)=0\]

    D) None of these

    Correct Answer: A

    Solution :

    Splitting the determinant into two determinants, we get \[\Delta =\left| \,\begin{matrix}    1 & a & {{a}^{2}}  \\    1 & b & {{b}^{2}}  \\    1 & c & {{c}^{2}}  \\ \end{matrix}\, \right|+abc\,\left| \,\begin{matrix}    1 & a & {{a}^{2}}  \\    1 & b & {{b}^{2}}  \\    1 & c & {{c}^{2}}  \\ \end{matrix}\, \right|\,=0\] = \[(1+abc)\,[(a-b)\,(b-c)\,(c-a)]=0\] Because a, b, c are different, the second factor cannot be zero. Hence, option (a), \[1+abc=\]0, is correct.


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