JEE Main & Advanced Mathematics Conic Sections Question Bank Ellipse

  • question_answer
    The value of \[\lambda \], for which the line \[2x-\frac{8}{3}\lambda y=-3\] is a normal to the conic \[{{x}^{2}}+\frac{{{y}^{2}}}{4}=1\] is [MP PET 2004]

    A)            \[\frac{\sqrt{3}}{2}\]                

    B)            \[\frac{1}{2}\]

    C)            \[-\frac{\sqrt{3}}{2}\]               

    D)            \[\frac{3}{8}\]

    Correct Answer: D

    Solution :

               We know that the equation of the normal at point \[(a\cos \theta ,\,b\sin \theta )\] on the curve \[{{x}^{2}}+\frac{{{y}^{2}}}{4}=1\] is given by            \[ax\sin \theta -by\text{cosec }\theta ={{a}^{2}}-{{b}^{2}}\]                            .....(i)            Comparing equation (i) with \[2x-\frac{8}{3}\lambda y=-3\]. We get,            \[a\sin \theta =2\], \[b\text{ cosec}\theta =\frac{8}{3}\lambda \] or \[ab=\frac{16}{3}\lambda \]   .....(ii)            \[\because \,a=1,\,b=2\]; \\[2=\frac{16}{3}\lambda \] or \[\lambda =3/8\]


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