JEE Main & Advanced Mathematics Vector Algebra Question Bank Vector triple product

  • question_answer
    Given the following simultaneous equations for vectors         x and y \[\mathbf{x}+\mathbf{y}=\mathbf{a}\]                    .....(i) \[\mathbf{x}\times \mathbf{y}=\mathbf{b}\]             .....(ii) \[\mathbf{x}\,.\,\mathbf{a}=1\]                                .....(iii) Then \[\mathbf{x}=.........,\,\,\,\mathbf{y}=.......\]                              [Roorkee 1994]

    A)          \[\mathbf{a},\,\,\mathbf{a}-\mathbf{x}\]

    B)          \[\mathbf{a}-\mathbf{b},\,\,\mathbf{b}\]

    C)          \[\mathbf{b},\,\,\mathbf{a}-b\]

    D)          None of these

    Correct Answer: D

    Solution :

               Multiplying (i) scalarly by \[\mathbf{a},\] we get \[\mathbf{a}\,.\,\mathbf{x}+\mathbf{a}\,.\,\mathbf{y}={{\mathbf{a}}^{2}}\]            \[\therefore \,\mathbf{a}\,.\,\mathbf{y}={{\mathbf{a}}^{2}}-1\]                 ?..(iv),    {By (iii)}            Again \[\mathbf{a}\times (\mathbf{x}\times \mathbf{y})=\mathbf{a}\times \mathbf{b}\]or \[(\mathbf{a}\,.\,\mathbf{y})\mathbf{x}-(\mathbf{a}\,.\,\mathbf{x})\mathbf{y}=\mathbf{a}\times \mathbf{b}\]            \[({{\mathbf{a}}^{2}}-1)\mathbf{x}-\mathbf{y}=\mathbf{a}\times \mathbf{b}\]   ?..(v),   {By (iii) and (iv)}            Adding and subtracting (i) and (v), we get \[x=\frac{\mathbf{a}+(\mathbf{a}\times \mathbf{b})}{{{\mathbf{a}}^{2}}}\] and \[\mathbf{y}=\mathbf{a}-\mathbf{x}\] etc.


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