Manipal Engineering Manipal Engineering Solved Paper-2010

  • question_answer
    If the matrix\[A=\left[ \begin{matrix}    y+a & b & c  \\    a & y+b & c  \\    a & b & y+c  \\ \end{matrix} \right]\]has rank 3, then

    A) \[y\ne (a-b+c)\]

    B) \[y\ne 1\]

    C) \[y=0\]

    D) \[y\ne -(a+b+c)\]and\[y\ne 0\]

    Correct Answer: D

    Solution :

    Here, the rank of\[A\]is 3. Therefore, the minor of order 3 of\[A\ne 0\] \[\Rightarrow \]               \[\left| \begin{matrix}    y+a & b & c  \\    a & y+b & c  \\    a & b & y+c  \\ \end{matrix} \right|\ne 0\] [Applying\[{{C}_{1}}\to {{C}_{1}}+{{C}_{2}}+{{C}_{3}}\], and taking\[(y+a=b+c)\]common from\[{{C}_{1}}\]] \[\Rightarrow \]               \[(y+a+b+c)\left| \begin{matrix}    1 & b & c  \\    1 & y+b & c  \\    1 & b & y+c  \\ \end{matrix} \right|\ne 0\] [Applying\[{{R}_{2}}\to {{R}_{2}}-{{R}_{1}},\,\,{{R}_{3}}\to {{R}_{3}}-{{R}_{1}}]\] \[\Rightarrow \]               \[(y+a+b+c)\left| \begin{matrix}    1 & b & c  \\    0 & y & 0  \\    0 & 0 & y  \\ \end{matrix} \right|\ne 0\] Expanding along \[{{C}_{1}}\] \[\Rightarrow \,\,\,(y+a+b+c)\,({{y}^{2}})\ne 0\] \[\Rightarrow \,\,y\ne 0\] and \[y\ne -(a+b+c)\]


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