JEE Main & Advanced Sample Paper JEE Main Sample Paper-30

  • question_answer
    If \[\left| {\vec{a}} \right|=\left| {\vec{b}} \right|=\left| {\vec{c}} \right|=2\] and \[\vec{a}.\vec{b}=\vec{b}.\vec{c}=\vec{c}.\vec{a}=2\], then \[[\vec{a}\,\vec{b}\,\vec{c}]\cos {{45}^{o}}\] is equal to

    A)  1                                

    B)  2

    C)  3                                

    D)  4

    Correct Answer: D

    Solution :

    \[{{[\vec{a}\vec{b}\vec{c}]}^{2}}=\left| \begin{matrix}    \vec{a}\cdot \vec{a} & \vec{a}\cdot \vec{b} & \vec{a}\cdot \vec{c}  \\    \vec{b}\cdot \vec{a} & \vec{b}\cdot \vec{b} & \vec{b}\cdot \vec{c}  \\    \vec{c}\cdot \vec{a} & \vec{c}\cdot \vec{b} & \vec{c}\cdot \vec{c}  \\ \end{matrix} \right|=\left| \begin{matrix}    4 & 2 & 2  \\    2 & 4 & 2  \\    2 & 2 & 4  \\ \end{matrix} \right|=32\] \[[\vec{a}\,\vec{b}\,\vec{c}]=4\sqrt{2}\Rightarrow [\vec{a}\,\vec{b}\,\vec{c}]\,\cos 45{}^\circ =4\]


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