RAJASTHAN ­ PET Rajasthan PET Solved Paper-2011

  • question_answer
    If a, b, c are non-zero non-collinear vectors and\[a\times b=b\times c=c\times a,\]then\[a+b+c\]is equal to

    A)  \[abc\]           

    B)  \[-1\]

    C)  2             

    D)  0

    Correct Answer: D

    Solution :

     Now, \[a\times b=b\times c\] \[\Rightarrow \] \[a\times b=-c\times b\] \[\Rightarrow \] \[(a+c)\times b=0\] \[\Rightarrow \] \[a+c||b\] \[\Rightarrow \]\[a+c=\lambda b\]for some scalar             ... (i) Further, \[c\times (a+c)=\lambda (c\times b)\] \[\Rightarrow \] \[(c\times a)+(c\times c)=\lambda c\times b\] \[\Rightarrow \] \[(c\times a)=-\lambda b\times c\] \[\Rightarrow \] \[(c\times a)=-\lambda (c\times a)\Rightarrow \lambda =-1\] Then from Eq. (i), \[a+c=-b\] \[\Rightarrow \] \[a+b+c=0\]


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