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BCECE Engineering BCECE Engineering Solved Paper-2007

  • question_answer
    If \[|\vec{a}|=3,\,\,\,|\vec{b}|=5\]and \[|\vec{c}|=4\]then and \[\vec{a}+\vec{b}+\vec{c}=0,\] then the value of \[\vec{a}+\vec{b}+\vec{c}=0,\]then the value of \[\vec{a}.\vec{b}\,+\,\vec{b}.\vec{c}\]is equal to

    A)  0                            

    B)         -25                        

    C)         25                         

    D)         none of these

    Correct Answer: B

    Solution :

    \[\because \]     \[|\vec{a}|=3,|\vec{b}|=5,|\vec{c}|=4\] and        \[\vec{a}+\vec{b}+\vec{c}=0\] \[\therefore \]  \[\vec{a}.\vec{b}+\vec{b}.b+\vec{c}.\vec{b}=\vec{0}\] \[\Rightarrow \]\[\vec{a}.\vec{b}+\vec{b}.\vec{c}=-|\vec{b}{{|}^{2}}\] \[\Rightarrow \]\[\vec{a}.\vec{b}+\vec{b}.\vec{c}=-25\]


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