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 \[{{x}^{2}}-xy-6{{y}^{2}}-7x+31y-18=0\] is  [Karnataka CET 2003]

    A)            \[{{45}^{o}}\]                       

    B)            \[{{60}^{o}}\]

    C)            \[{{90}^{o}}\]                       

    D)            \[{{30}^{o}}\]

    Correct Answer: A

    Solution :

               Comparing given equation with \[a{{x}^{2}}+2hxy+b{{y}^{2}}+2gx+2fy+c=0\] we get \[a=1,\,h=-1/2,\,b=-6,g=-\frac{7}{2},\,f=\frac{31}{2},\,c=-18\]            Now angle between the lines            \[\theta ={{\tan }^{-1}}\left| \,\frac{2\sqrt{{{\left( \frac{-1}{2} \right)}^{2}}-1\times (-6)}}{1+(-6)}\, \right|={{\tan }^{-1}}\left| \,\frac{2\sqrt{\frac{1}{4}+6}}{-5}\, \right|\]               \[={{\tan }^{-1}}\left| \,\frac{\left( 2\times \frac{5}{2} \right)}{-5} \right|\]=\[{{\tan }^{1}}|-1|\] = \[{{\tan }^{-1}}(1)\]=\[\frac{\pi }{4}\] or \[{{45}^{o}}\].


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