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

  • question_answer
    The equation \[2{{x}^{2}}+4xy-k{{y}^{2}}+4x+2y-1=0\] represents a pair of lines.  The value of k is [Karnataka CET 2001]

    A)            \[-\frac{5}{3}\]                    

    B)            \[\frac{5}{3}\]

    C)            \[\frac{1}{3}\]                      

    D)            \[-\frac{1}{3}\]

    Correct Answer: A

    Solution :

               Comparing the given equation with the standard equation, we get \[a=2,\ f=1,\ b=-k\], \[g=2,\] \[c=-1\] and \[h=2\]. Since equation represents a pair of lines, therefore \[abc+2fgh-a{{f}^{2}}-b{{g}^{2}}-c{{h}^{2}}=0\]or \[2\,k+8-2+4k+4=0\] or \[6k=-10\]. Therefore \[k=-\frac{5}{3}\].


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