JEE Main & Advanced Mathematics Pair of Straight Lines Question Bank Equation of Pair of Straight Lines

  • question_answer
    The condition of representing the coincident lines by the general quadratic equation \[f(x,\,y)=0\], is

    A) \[\Delta =0\] and \[{{h}^{2}}=ab\]

    B) \[\Delta =0\] and \[a+b=0\]  

    C) \[\Delta =0\] and \[{{h}^{2}}=ab\], \[{{g}^{2}}=ac\], \[{{f}^{2}}=bc\]

    D) \[{{h}^{2}}=ab\], \[{{g}^{2}}=ac\] and \[{{f}^{2}}=bc\]

    Correct Answer: B , C

    Solution :

    Comparing the given equation with the standard equation, we get \[a=4\] and\[b=-7\]. Let \[{{m}_{1}}\] and \[{{m}_{2}}\] are the slopes of given lines. Therefore sum of the slopes \[({{m}_{1}}+{{m}_{2}})=-\frac{2h}{b}=\frac{2h}{7}\] and product of the slopes\[({{m}_{1}}{{m}_{2}})=\frac{a}{b}=\frac{4}{-7}\].                     \[\because {{m}_{1}}+{{m}_{2}}={{m}_{1}}{{m}_{2}}\], therefore \[\frac{2h}{7}=\frac{4}{-7}\] or \[h=-2\].

    Solution :

    It is obvious.


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