JEE Main & Advanced Mathematics Determinants & Matrices Question Bank Minors and Co-factors, Product of determinants

  • question_answer
    If \[\Delta =\left| \,\begin{matrix}    {{a}_{1}} & {{b}_{1}} & {{c}_{1}}  \\    {{a}_{2}} & {{b}_{2}} & {{c}_{2}}  \\    {{a}_{3}} & {{b}_{3}} & {{c}_{3}}  \\ \end{matrix}\, \right|\] and \[{{A}_{1}},{{B}_{1}},{{C}_{1}}\]denote the co-factors of \[{{a}_{1}},{{b}_{1}},{{c}_{1}}\] respectively, then the value of the determinant \[\left| \begin{matrix}    {{A}_{1}} & {{B}_{1}} & {{C}_{1}}  \\    {{A}_{2}} & {{B}_{2}} & {{C}_{2}}  \\    {{A}_{3}} & {{B}_{3}} & {{C}_{3}}  \\ \end{matrix} \right|\] is [MP PET 1989]

    A) \[\Delta \]

    B) \[{{\Delta }^{2}}\]

    C) \[{{\Delta }^{3}}\]

    D) 0

    Correct Answer: B

    Solution :

    We know that \[\Delta \,{\Delta }'=\left| \,\begin{matrix}    {{a}_{1}} & {{b}_{1}} & {{c}_{1}}  \\    {{a}_{2}} & {{b}_{2}} & {{c}_{2}}  \\    {{a}_{3}} & {{b}_{3}} & {{c}_{3}}  \\ \end{matrix}\, \right|.\left| \,\begin{matrix}    {{A}_{1}} & {{B}_{1}} & {{C}_{1}}  \\    {{A}_{2}} & {{B}_{2}} & {{C}_{2}}  \\    {{A}_{3}} & {{B}_{3}} & {{C}_{3}}  \\ \end{matrix}\, \right|\] \[=\left| \,\begin{matrix}    \Sigma {{a}_{1}}{{A}_{1}} & 0 & 0  \\    0 & \Sigma {{a}_{2}}{{A}_{2}} & 0  \\    0 & 0 & \Sigma {{a}_{3}}{{A}_{3}}  \\ \end{matrix}\, \right|=\left| \,\begin{matrix}    \Delta  & 0 & 0  \\    0 & \Delta  & 0  \\    0 & 0 & \Delta   \\ \end{matrix}\, \right|={{\Delta }^{3}}\] Þ \[{\Delta }'={{\Delta }^{2}}\].


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