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

  • question_answer
    For what value of \['p'\], \[{{y}^{2}}+xy+p{{x}^{2}}-x-2y=0\] represents two straight lines                                            [UPSEAT 2002]

    A)            2    

    B)            \[\frac{1}{3}\]

    C)            \[\frac{1}{4}\]                      

    D)            \[\frac{1}{2}\]

    Correct Answer: C

    Solution :

               For this to represent straight lines                    \[abc+2fgh-a{{f}^{2}}-b{{g}^{2}}-c{{h}^{2}}=0\]                    Þ \[{{p}^{2}}+2(-1)\left( \frac{-1}{2} \right)\,\left( \frac{1}{2} \right)-p(1)-1\left( \frac{1}{4} \right)-p\left( \frac{1}{4} \right)\]                    Þ \[{{p}^{2}}+\frac{1}{2}-p-\frac{1}{4}-\frac{p}{4}=0\]Þ \[{{p}^{2}}-\frac{5p}{4}+\frac{1}{4}=0\]                    Þ \[4{{p}^{2}}-5p+1=0\] Þ \[4{{p}^{2}}-4p-p+1=0\]                    Þ \[(4p-1)(p-1)=0\] Þ \[p=\frac{1}{4},\,1.\]


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