JEE Main & Advanced Mathematics Pair of Straight Lines Question Bank Bisectors of the angle between the lines, Point of intersection of the lines

  • question_answer
    If \[y=mx\]be one of the bisectors of the angle between the lines \[a{{x}^{2}}-2hxy+b{{y}^{2}}=0\], then

    A)            \[h\,(1+{{m}^{2}})+m(a-b)=0\]

    B)            \[h\,(1-{{m}^{2}})+m(a+b)=0\]

    C)            \[h\,(1-{{m}^{2}})+m(a-b)=0\]

    D)            \[h\,(1+{{m}^{2}})+m(a+b)=0\]

    Correct Answer: C

    Solution :

               Here equation of one bisector of angle is \[y-mx=0,\] therefore equation of second is \[x+my=0\].            Hence combined equation is \[(x+my)(y-mx)=0\]            \[\Rightarrow -m{{x}^{2}}-xy({{m}^{2}}-1)+m{{y}^{2}}=0\]                               .?.(i)            Also equations of bisectors of \[a{{x}^{2}}-2hxy+b{{y}^{2}}=0\] is            \[-h{{x}^{2}}-(a-b)xy+h{{y}^{2}}=0\]                            .....(ii)            Hence (i) and (ii) are the same equations, therefore            \[\frac{m}{h}=\frac{{{m}^{2}}-1}{(a-b)}\Rightarrow h({{m}^{2}}-1)=m(a-b)\]                      \[\Rightarrow m(a-b)+h(1-{{m}^{2}})=0\].


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