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

  • question_answer
    If the slope of one of the line represented by the equation \[a{{x}^{2}}+2hxy+b{{y}^{2}}=0\] be l times that of the other, then

    A)            \[4\lambda h=ab(1+\lambda )\]                                     

    B)            \[\lambda h=ab{{(1+\lambda )}^{2}}\]

    C)            \[4\lambda {{h}^{2}}=ab{{(1+\lambda )}^{2}}\]       

    D)            None of these

    Correct Answer: C

    Solution :

               It is given that \[{{m}_{2}}=\lambda {{m}_{1}}\Rightarrow {{m}_{1}}+\lambda {{m}_{1}}=\frac{-2h}{b}\]                    \[\Rightarrow {{m}_{1}}=\frac{-2h}{b(1+\lambda )}\]                                   .....(i)                    and \[{{m}_{1}}.\lambda {{m}_{1}}=\frac{a}{b}\Rightarrow {{m}_{1}}=\sqrt{\frac{a}{b\lambda }}\]                   ..?(ii)                    Hence, by (i) and (ii), \[\sqrt{\frac{a}{b\lambda }}=\frac{-2h}{b(1+\lambda )}\]                    On squaring both sides, we get \[4\lambda {{h}^{2}}=ab{{(1+\lambda )}^{2}}\].


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