CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2007

  • question_answer
    Let a, b and c be non-zero vectors such that\[(\overrightarrow{a}.\overrightarrow{b})\times \overrightarrow{c}=\frac{-1}{4}|\overrightarrow{b}||\overrightarrow{c}|\overrightarrow{a}.\]. If\[\theta \]is the acute angle between the vectors \[\vec{b}\] and \[\vec{c}\], then the angle between\[\overrightarrow{a}\]and\[\overrightarrow{c}\]is equal to

    A)  \[\frac{2\pi }{3}\]                                           

    B)  \[\frac{\pi }{4}\]

    C)  \[\frac{\pi }{3}\]                             

    D)         \[\frac{\pi }{2}\]

    E)  \[\frac{\pi }{6}\]

    Correct Answer: D

    Solution :

    Given that \[(\overrightarrow{a}\times \overrightarrow{b})\times \overrightarrow{c}=-\frac{1}{4}|\overrightarrow{b}||\overrightarrow{c}|\overrightarrow{a}\] \[\Rightarrow \] \[(\overrightarrow{c}.\overrightarrow{a})\overrightarrow{b}-(\overrightarrow{c}.\overrightarrow{b})\overrightarrow{a}=-\frac{1}{4}|\overrightarrow{b}||\overrightarrow{c}|\overrightarrow{a}\] \[\Rightarrow \]               \[(\overrightarrow{c}.\overrightarrow{a})\overrightarrow{b}=0\] \[\Rightarrow \] \[|\overrightarrow{c}||\overrightarrow{a}|\cos \theta =0\Rightarrow \theta =\frac{\pi }{2}\]


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