JEE Main & Advanced Mathematics Pair of Straight Lines Question Bank Angle between the pair of straight lines, Condition for parallel and perpendicular lines

  • question_answer
    . If \[(a+3b)(3a+b)=4{{h}^{2}}\], then the angle between the lines represented by \[a{{x}^{2}}+2hxy+b{{y}^{2}}=0\] is

    A)            \[{{30}^{o}}\]                       

    B)            \[{{45}^{o}}\]

    C)            \[{{60}^{o}}\]                       

    D)            \[{{\tan }^{-1}}\frac{1}{2}\]

    Correct Answer: C

    Solution :

               \[\theta ={{\tan }^{-1}}\left( \frac{2\sqrt{{{h}^{2}}-ab}}{a+b} \right)={{\tan }^{-1}}\left( \frac{\sqrt{4{{h}^{2}}-4ab}}{a+b} \right)\]               \[={{\tan }^{-1}}\left( \frac{\sqrt{3{{a}^{2}}+3{{b}^{2}}+10ab-4ab}}{a+b} \right)=60{}^\circ \].


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