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 the lines represented by the equation \[a{{x}^{2}}-bxy-{{y}^{2}}=0\] make angles \[\alpha \] and \[\beta \] with the x-axis, then \[\tan (\alpha +\beta )\]=

    A)            \[\frac{b}{1+a}\].               

    B)            \[\frac{-b}{1+a}\]

    C)            \[\frac{a}{1+b}\]                

    D)            None of these

    Correct Answer: B

    Solution :

                 Here the equation is \[a{{x}^{2}}-bxy-{{y}^{2}}=0\] and given that \[{{m}_{1}}=\tan \alpha \] and \[{{m}_{2}}=\tan \beta \]and we know that            \[{{m}_{1}}+{{m}_{2}}=\frac{b}{-1}=\tan \alpha +\tan \beta \]            and \[{{m}_{1}}{{m}_{2}}=\frac{a}{-1}=\tan \alpha .\tan \beta \]            \[\tan (\alpha +\beta )=\frac{\tan \alpha +\tan \beta }{1-\tan \alpha \tan \beta }=\frac{-b}{1-(-a)}=\frac{-b}{(1+a)}\].


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