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

  • question_answer
    If \[\mathbf{a}=\mathbf{i}-\mathbf{j},\mathbf{b}=\mathbf{i}+\mathbf{j},\,\,\,\mathbf{c}=\mathbf{i}+3\mathbf{j}+5\mathbf{k}\]and\[\mathbf{n}\]is a unit vector such that \[\mathbf{b}.\mathbf{n}=0,\mathbf{a}.\mathbf{n}=0\]then the value of \[|\mathbf{c}\ .\ \mathbf{n}|\] is equal to [DCE 2005]

    A)                 1

    B)                 3

    C)             5

    D)                 2

    Correct Answer: C

    Solution :

               \ n is perpendicular to \[\mathbf{a}\] and \[\mathbf{b}\]                    \[\mathbf{n}=\mathbf{a}\times \mathbf{b}\]; \[\mathbf{n}=\]\[\frac{\left| \begin{matrix}    \mathbf{i} & \mathbf{j} & \mathbf{k}  \\    1 & -1 & 0  \\    1 & 1 & 0  \\ \end{matrix} \right|}{\sqrt{2}\times \sqrt{2}}\] = \[\frac{2\mathbf{k}}{2}=\mathbf{k}\]                                 \ \[|\mathbf{c}\ .\ \mathbf{n}|\ =\,|(\mathbf{i}+3\mathbf{j}+5\mathbf{k}).(\mathbf{k})|\,=\ |5|\,=5\].


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