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

  • question_answer
    If the vectors a and b are mutually perpendicular, then \[\mathbf{a}\times \{\mathbf{a}\times \{\mathbf{a}\times (\mathbf{a}\times \mathbf{b})\}\}\] is equal to

    A)          \[|\mathbf{a}{{|}^{2}}\mathbf{b}\]

    B)          \[|\mathbf{a}{{|}^{3}}\mathbf{b}\]

    C)          \[|\mathbf{a}{{|}^{4}}\mathbf{b}\]

    D)          None of these

    Correct Answer: C

    Solution :

               \[\mathbf{a}\times [\mathbf{a}\times \{\mathbf{a}\times (\mathbf{a}\times \mathbf{b})\}]=\mathbf{a}\times [\mathbf{a}\times \{\mathbf{a}\times ab\,\mathbf{\hat{n}}\}]\]                    \[=\mathbf{a}\times [\mathbf{a}\times {{a}^{2}}\mathbf{b}]=\mathbf{a}\times {{a}^{3}}b\,\mathbf{\hat{n}}=|\mathbf{a}{{|}^{4}}\mathbf{b}.\]


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