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
    . The angle between the lines represented by the equation \[a{{x}^{2}}+xy+b{{y}^{2}}=0\] will be \[{{45}^{o}}\], if

    A)            \[a=1,b=6\]                          

    B)            \[a=1,b=-6\]

    C)            \[a=6,b=1\]                          

    D)            None of these

    Correct Answer: B

    Solution :

               \[\tan 45{}^\circ =\frac{2\sqrt{\frac{1}{4}-ab}}{a+b}\]            \[\Rightarrow {{(a+b)}^{2}}=(1-4ab)\Rightarrow {{a}^{2}}+{{b}^{2}}+6ab-1=0\],            which is obviously satisfied by \[a=1\]and \[b=-6\].


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