JEE Main & Advanced Mathematics Vector Algebra Question Bank Vector or Cross product of two vectors and its application

  • question_answer
    If \[\mathbf{a}\times \mathbf{b}=\mathbf{b}\times \mathbf{c}\ne 0,\] where a, b and c are coplanar vectors, then for some scalar k                    [Roorkee 1985; RPET 1997]

    A)                 \[\mathbf{a}+\mathbf{c}=k\,\mathbf{b}\]

    B)                 \[\mathbf{a}+\mathbf{b}=k\,\mathbf{c}\]

    C)                 \[\mathbf{b}+\mathbf{c}=k\,\mathbf{a}\]

    D)                 None of these

    Correct Answer: A

    Solution :

                       Since \[\mathbf{a}\times \mathbf{b}=\mathbf{b}\times \mathbf{c}\ne \mathbf{0}\Rightarrow \mathbf{a}\times \mathbf{b}-\mathbf{b}\times \mathbf{c}=\mathbf{0}\]                    \[\Rightarrow \mathbf{a}\times \mathbf{b}+\mathbf{c}\times \mathbf{b}=\mathbf{0}\Rightarrow (\mathbf{a}+\mathbf{c})\times \mathbf{b}=\mathbf{0}\]                                 \[\Rightarrow \mathbf{a}+\mathbf{c}\] is parallel to \[\mathbf{b}\Rightarrow \mathbf{a}+\mathbf{c}=k\mathbf{b}.\]


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