JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2006

  • question_answer
        Angle between the vectors\[\sqrt{3}(\overrightarrow{a}\times \overrightarrow{b})\]and\[\overrightarrow{b}-(\overrightarrow{a}.\overrightarrow{b})\overrightarrow{a}\]is

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

    B)  0   

    C)  \[\frac{\pi }{4}\]                             

    D)  \[\frac{\pi }{3}\]

    Correct Answer: A

    Solution :

                    Let \[\overrightarrow{A}=\sqrt{3}(\overrightarrow{a}\times \overrightarrow{b}),\overrightarrow{B}=\overrightarrow{b}-(\overrightarrow{a}.\overrightarrow{b})\overrightarrow{a}\] Now, \[\overrightarrow{A}-\overrightarrow{B}=0\] \[\Rightarrow \]Angle between\[\overrightarrow{A}\]and\[\overrightarrow{B}\]is\[\frac{\pi }{2}\]. Alternative Method \[\overrightarrow{A}\]is lector\[\bot \]to plane of\[\overrightarrow{a}\]and\[\overrightarrow{b}\]while\[\overrightarrow{B}\]lies in  he plane of\[\overrightarrow{a}\]and\[\overrightarrow{b}\]. \[\Rightarrow \]               \[\overrightarrow{A}\bot \overrightarrow{B}\] \[\therefore \]Angle between\[\overrightarrow{A}\]and\[\overrightarrow{B}\]is\[\frac{\pi }{2}\].


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