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

  • question_answer
    The value of \[\lambda ,\] for which the equation \[{{x}^{2}}-{{y}^{2}}-x\]?\[\lambda y-2=0\]represent a pair of straight line, are [MP PET 2004]

    A)            3, - 3                                        

    B)            - 3, 1                                        

    C)            3, 1

    D)            -1, 1

    Correct Answer: A

    Solution :

               Given equation is\[{{x}^{2}}-{{y}^{2}}-x-\lambda y-2=0\].            Here, \[a=1\], \[b=-1\], \[c=-2\], \[h=0\], \[g=\frac{-1}{2}\], \[f=\frac{-\lambda }{2}\]            This equation represents a pair of straight line, if \[abc+2fgh-a{{f}^{2}}-b{{g}^{2}}-c{{h}^{2}}=0\]            \[2-\frac{{{\lambda }^{2}}}{4}+\frac{1}{4}=0\] Þ \[\frac{{{\lambda }^{2}}}{4}=\frac{9}{4}\Rightarrow {{\lambda }^{2}}=9,\,\lambda =\pm 3,\]i.e., (3, ? 3).


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