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

  • question_answer
    The equation \[4{{x}^{2}}+12xy+9{{y}^{2}}+2gx+2fy+c=0\] will represents two real parallel straight lines, if

    A)            g = 4, f = 9, c = 0                  

    B)            g = 2, f = 3, c = 1

    C)            g = 2, f = 3, c is any number

    D)            g = 4, f = 9, c > 1

    Correct Answer: C

    Solution :

               The lines are parallel, if \[{{h}^{2}}=ab,\,a{{f}^{2}}=b{{g}^{2}}\]                    or \[\frac{a}{h}=\frac{h}{b}=\frac{g}{f}\Rightarrow 4{{f}^{2}}=9{{g}^{2}}\]                    \[\Rightarrow f=\frac{3}{2}g\Rightarrow g=2,\ \ f=3\] (let)                    Now \[abc+2fgh-a{{f}^{2}}-b{{g}^{2}}-c{{h}^{2}}=0\]                    \[\Rightarrow 4\times 9\times c+2\times 3\times 2\times 6-4{{(3)}^{2}}-9{{(2)}^{2}}-c{{(6)}^{2}}=0\]                    \[\Rightarrow \]c is any number.


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