BCECE Engineering BCECE Engineering Solved Paper-2007

  • question_answer
    If \[A=\left[ \begin{matrix}    1 & x  \\    {{x}^{2}} & 4y  \\ \end{matrix} \right]\]and \[B\,=\left[ \begin{matrix}    -3 & 1  \\    1 & 0  \\ \end{matrix} \right]\]adj.\[A+B=\left[ \begin{matrix}    1 & 0  \\    0 & 1  \\ \end{matrix} \right],\]  then the value of \[x\]and \[y\]are

    A)  1, 1                       

    B)         \[\pm \,1,1\]    

    C)         \[1,0\]                 

    D)         none of these

    Correct Answer: A

    Solution :

    \[\because \]     \[A=\left[ \begin{matrix}    1 & x  \\    {{x}^{2}} & 4y  \\ \end{matrix} \right]\]and \[B=\left[ \begin{matrix}    -3 & 1  \\    1 & 0  \\ \end{matrix} \right]\]                                 \[\text{Adj}\,\text{A}\,\text{=}\,\left[ \begin{matrix}    4y & -x  \\    -{{x}^{2}} & 1  \\ \end{matrix} \right]\]                 \[\therefore \]  \[\text{adj}\,\text{+}\,A+B=\left[ \begin{matrix}    4y & -x  \\    -{{x}^{2}} & 1  \\ \end{matrix} \right]+\left[ \begin{matrix}    -3 & 1  \\    1 & 0  \\ \end{matrix} \right]\]                 \[\Rightarrow \]               \[\left[ \begin{matrix}    1 & 0  \\    0 & 1  \\ \end{matrix} \right]=\left[ \begin{matrix}    4y-3 & -x+1  \\    -{{x}^{2}}+1 & 1+0  \\ \end{matrix} \right]\]                 \[\Rightarrow \]               \[4y-3=1\Rightarrow y=1\]                 and        \[-x+1=0\Rightarrow x=1\]


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