JEE Main & Advanced Mathematics Vector Algebra Question Bank Scalar triple product and their applications

  • question_answer
    If \[\mathbf{a,}\,\mathbf{b,}\,\mathbf{c}\] are non-coplanar vectors and \[\mathbf{d}=\lambda \mathbf{a}+\mu \,\mathbf{b}+\nu \mathbf{c},\] then \[\lambda \] is equal to [Roorkee 1999]

    A)             \[\frac{[\mathbf{d}\,\mathbf{b}\,\mathbf{c}]}{[\mathbf{b}\,\mathbf{a}\,\mathbf{c}]}\]

    B)             \[\frac{[\mathbf{b}\,\mathbf{c}\,\mathbf{d}]}{[\mathbf{b}\,\mathbf{c}\,\mathbf{a}]}\]

    C)             \[\frac{[\mathbf{b}\,\mathbf{d}\,\mathbf{c}]}{[\mathbf{a}\,\mathbf{b}\,\mathbf{c}]}\]

    D)             \[\frac{[\mathbf{c}\,\mathbf{b}\,\mathbf{d}\,]}{[\mathbf{a}\,\mathbf{b}\,\mathbf{c}]}\]

    Correct Answer: B

    Solution :

                    Since \[d=\lambda a+\mu b+\nu c\]                 \\[d.(b\times c)=\lambda \,a.(b\times c)+\mu \,b.(b\times c)+\mu \,c.(b\times c)\]                 \[=\lambda \left[ a\,b\,c \right]\]                  Þ \[\lambda =\frac{[d\,b\,c]}{[a\,b\,c]}\]\[=\frac{[b\,c\,d]}{[b\,c\,a]}\].b


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