JEE Main & Advanced JEE Main Paper (Held On 12-Jan-2019 Morning)

  • question_answer
    An ordered pair \[(\alpha ,\beta )\]for which the system of linear equations \[(1+\alpha )x+\beta y+z=2,\]\[\alpha x+(1+\beta )y+z=3,\]\[\alpha x+\beta y+2z=2\] has a unique solution, is [JEE Main Online Paper Held On 12-Jan-2019 Morning]

    A) \[(-3,\text{ }1)\]                         

    B) \[(2,\text{ 4})\]

    C) \[(1,\text{ }-3)\]             

    D)   \[(-4,\text{ 2})\]

    Correct Answer: B

    Solution :

    For unique solution\[\left| \begin{matrix}    1+\alpha  & \beta  & 1  \\    \alpha  & 1+\beta  & 1  \\    \alpha  & \beta  & 2  \\ \end{matrix} \right|\ne 0\] \[\Rightarrow \]\[(1+\alpha )(2+2\beta -\beta )-\beta (2\alpha -\alpha )+1(\alpha \beta -\alpha -\alpha \beta )\ne 0\] \[\Rightarrow \]\[2+2\beta -\beta +2\alpha +2\alpha \beta -\alpha \beta -2\alpha \beta +\alpha \beta -\alpha \ne 0\] \[\Rightarrow \]\[2+\alpha +\beta \ne 0\] \[\therefore \](2, 4) satisfies the above condition.


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