BCECE Engineering BCECE Engineering Solved Paper-2012

  • question_answer
    The angle between the lines whose  direction cosines are \[\left( \frac{\sqrt{3}}{4},\frac{1}{4},\frac{\sqrt{3}}{2} \right)\] and \[\left( \frac{\sqrt{3}}{4},\frac{1}{4}-\frac{\sqrt{3}}{2} \right)\]is

    A) \[\pi \]                                 

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

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

    D)         \[\frac{\pi }{4}\]

    Correct Answer: C

    Solution :

    \[\cos \theta =|{{l}_{1}}{{l}_{2}}+{{m}_{1}}{{m}_{2}}+{{n}_{1}}{{n}_{2}}|\] \[=\left| \frac{\sqrt{3}}{4}\times \frac{\sqrt{3}}{4}+\frac{1}{4}\times \frac{1}{4}+\frac{\sqrt{3}}{2}\times \left( \frac{-\sqrt{3}}{2} \right) \right|\] \[=\left| \frac{3}{16}+\frac{1}{16}-\frac{3}{4} \right|\] \[=\left| \frac{-2}{4} \right|=\frac{1}{2}\]                 \[\Rightarrow \]               \[\theta =\frac{\pi }{3}\]


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