Manipal Engineering Manipal Engineering Solved Paper-2010

  • question_answer
    If   the   equation \[l{{x}^{2}}+2mxy+n{{y}^{2}}=0\]represents a pair conjugate diameter of the hyperbola\[\frac{{{x}^{2}}}{{{a}^{2}}}-\frac{{{y}^{2}}}{{{b}^{2}}}=1\], then

    A) \[l{{a}^{2}}+n{{b}^{2}}=0\]                         

    B) \[l{{a}^{2}}=n{{b}^{2}}\]

    C) \[2l{{a}^{2}}=n{{b}^{2}}\]            

    D)         None of these

    Correct Answer: B

    Solution :

    Let\[y={{m}_{1}}x\]and\[y={{m}_{2}}x\]be the lines represented by\[l{{x}^{2}}+2mxy+n{{y}^{2}}=0\], then                 \[{{m}_{1}}{{m}_{2}}=\frac{1}{n}\]                                           ... (i) But\[y={{m}_{1}}x\]and\[y={{m}_{2}}x\]are conjugate diameters of the hyperbola. \[\therefore \]  \[{{m}_{1}}{{m}_{2}}=\frac{{{b}^{2}}}{{{a}^{2}}}\]                                             ... (ii) From Eqs. (i) and (ii), we get                 \[\frac{l}{n}=\frac{{{b}^{2}}}{{{a}^{2}}}\Rightarrow l{{a}^{2}}=n{{b}^{2}}\]


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