JEE Main & Advanced Sample Paper JEE Main Sample Paper-24

  • question_answer
    Consider the matrix \[A\left( \begin{matrix}    3 & -2  \\    4 & -1  \\ \end{matrix} \right)\]. Then all possible values of \[\lambda \] such that the determinant of \[B=A-\lambda \,I\] is 0, where \[I=\left( \begin{matrix}   1 & 0  \\    0 & 1  \\ \end{matrix} \right)\]and \[i=\sqrt{-1}\]

    A)  \[1\pm 2\,i\]                             

    B)  \[2\pm 3\,i\]

    C)  \[3\pm 4\,i\]                             

    D)  \[5\pm 6\,i\]

    Correct Answer: C

    Solution :

    We have \[B=A-\lambda I\,=\left( \begin{matrix}    3 & -2  \\    4 & -1  \\ \end{matrix} \right)\,-\left( \begin{matrix}    \lambda  & 0  \\    0 & \lambda   \\ \end{matrix} \right)\] \[=\left( \begin{matrix}    3-\lambda  & -2  \\    4 & -1-\lambda   \\ \end{matrix} \right)\] Thus det \[=(3-\lambda )\,(-1-\lambda )\,-(-2)\,4=-3\] \[-3\lambda +\lambda +{{\lambda }^{2}}+8\] \[={{\lambda }^{2}}\,-2\lambda +5=0\] Solving gives \[\lambda =\frac{2\pm \,4i}{2}\,=1\pm \,2i\]


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