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

  • question_answer
    If the equation \[hxy+gx+fy+c=0\] represents a pair of straight lines, then

    A)            \[fh=cg\]                                   

    B)            \[fg=ch\]

    C)            \[{{h}^{2}}=gf\]                         

    D)            \[fgh=c\]

    Correct Answer: B

    Solution :

               \[hxy+gx+fy+c=0,\]Apply \[\Delta '=0\]                    i.e., \[a'b'c'+2f'g'h'-a'f{{'}^{2}}-b'g{{'}^{2}}-{c}'\,h{{'}^{2}}=0\]                    Here \[h'=\frac{h}{2},\ g'=\frac{g}{2},\ f'=\frac{f}{2},\ c'=c,\ a'=0,\ b'=0\]                    Hence, \[\frac{hgf}{4}-\frac{c{{h}^{2}}}{4}=0\]or\[fg=ch\].


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