RAJASTHAN ­ PET Rajasthan PET Solved Paper-2007

  • question_answer
    If\[\overrightarrow{a}=\hat{i}-\hat{k},\overrightarrow{b}=x\hat{i}+\hat{j}+(1-x)\hat{k}\]and\[\overrightarrow{c}=y\hat{i}+x\hat{j}+(1+x-y)\hat{k},\]then\[[\overrightarrow{a}\text{ }\overrightarrow{b}\text{ }\overrightarrow{c}]\]depends on

    A)  only y            

    B)  only\[x\]

    C)  both\[x\]and y     

    D)  neither\[x\] nor y

    Correct Answer: D

    Solution :

     \[[\overrightarrow{a}\,\overrightarrow{b}\,\overrightarrow{c}]=\left| \begin{matrix}    1 & 0 & -1  \\    x & 1 & 1-x  \\    y & x & 1+x-y  \\ \end{matrix} \right|\] Applying \[{{C}_{3}}\to {{C}_{3}}+{{C}_{1}}\] \[=\left| \begin{matrix}    1 & 0 & 0  \\    x & 1 & 1  \\    y & x & 1+x  \\ \end{matrix} \right|\] \[=1[1+x-x]=1\] Hence,\[[\overrightarrow{a}\overrightarrow{b}\overrightarrow{c}]\]neither depends on\[x\]nor y.


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