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 lines represented by \[a{{x}^{2}}+2hxy+b{{y}^{2}}=0\] be the square of the other, then

    A)            \[{{a}^{2}}b+a{{b}^{2}}-6abh+8{{h}^{3}}=0\]

    B)            \[{{a}^{2}}b+a{{b}^{2}}+6abh+8{{h}^{3}}=0\]

    C)            \[{{a}^{2}}b+a{{b}^{2}}-3abh+8{{h}^{3}}=0\]

    D)            \[{{a}^{2}}b+a{{b}^{2}}-6abh-8{{h}^{3}}=0\]

    Correct Answer: A

    Solution :

               Here,  \[{{m}_{1}}=m_{2}^{2}\Rightarrow m_{2}^{2}+{{m}_{2}}=\frac{-2h}{b}\]                      ..... (i)                    and \[m_{2}^{2}{{m}_{2}}=\frac{a}{b}\Rightarrow {{m}_{2}}={{\left( \frac{a}{b} \right)}^{1/3}}\]                       .....(ii)                    Putting this value of \[{{m}_{2}}\]in (i), we get                    \[{{\left\{ {{\left( \frac{a}{b} \right)}^{1/3}} \right\}}^{2}}+{{\left( \frac{a}{b} \right)}^{1/3}}=\frac{-2h}{b}\]                    On cubing both sides, we get                    \[{{\left( \frac{a}{b} \right)}^{2}}+\frac{a}{b}+3{{\left( \frac{a}{b} \right)}^{2/3}}.{{\left( \frac{a}{b} \right)}^{1/3}}.\left\{ {{\left( \frac{a}{b} \right)}^{2/3}}+{{\left( \frac{a}{b} \right)}^{1/3}} \right\}=\frac{-8{{h}^{3}}}{{{b}^{3}}}\]                    Þ\[{{\left( \frac{a}{b} \right)}^{2}}+\frac{a}{b}-\frac{6ah}{{{b}^{2}}}=\frac{-8{{h}^{3}}}{{{b}^{3}}}\] \[\left\{ \because {{\left( \frac{a}{b} \right)}^{2/3}}+{{\left( \frac{a}{b} \right)}^{1/3}}=\frac{-2h}{b} \right\}\]                    Þ \[ab(a+b)-6abh+8{{h}^{3}}=0\].


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