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
    Angle between the line joining the origin to the points of intersection of the curves \[2{{x}^{2}}+3{{y}^{2}}+10x=0\] and \[3{{x}^{2}}+5{{y}^{2}}+16x=0\] is

    A)            \[{{\tan }^{-1}}\frac{3}{2}\]   

    B)            \[{{\tan }^{-1}}\frac{4}{5}\]

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

    D)            None of these

    Correct Answer: C

    Solution :

               The equation of any curve through the points of intersection of the given curves is            \[2{{x}^{2}}+3{{y}^{2}}+10x+\lambda (3{{x}^{2}}+5{{y}^{2}}+16x)=0\]         .....(i)            If this equation represents two straight lines through the origin, then this must be homogeneous equation of second degree i.e., coefficient of x in (i) must vanish            \ \[10+16\lambda =0\Rightarrow \lambda =\frac{-10}{16}=\frac{-5}{8}\]            Substituting this value of \[\lambda \]in (i), we get the equation of pair of straight lines \[{{x}^{2}}-{{y}^{2}}=0\]               ?..(ii)                    Hence the lines represented by the equation (ii) are mutually perpendicular.


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