JEE Main & Advanced Mathematics Conic Sections Question Bank Hyperbola

  • question_answer
    The value of m, for which the line \[y=mx+\frac{25\sqrt{3}}{3}\], is a normal  to the conic \[\frac{{{x}^{2}}}{16}-\frac{{{y}^{2}}}{9}=1\], is [MP PET 2004]

    A)            \[\sqrt{3}\]                               

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

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

    D)            1

    Correct Answer: B

    Solution :

                      We know that the equation of the normal of the conic \[\frac{{{x}^{2}}}{{{a}^{2}}}-\frac{{{y}^{2}}}{{{b}^{2}}}=1\] at point \[(a\sec \theta ,\,b\tan \theta )\] is \[ax\sec \theta +by\cot \theta ={{a}^{2}}+{{b}^{2}}\]                   or \[y=\frac{-a}{b}\sin \theta \,x+\frac{{{a}^{2}}+{{b}^{2}}}{b\cot \theta }\]            Comparing above equation with equation \[y=mx+\frac{25\sqrt{3}}{3}\] and taking \[a=4,\,b=3\]            we get, \[\frac{{{a}^{2}}+{{b}^{2}}}{b\cot \theta }=\frac{25\sqrt{3}}{3}\] \[\Rightarrow \]\[\tan \theta =\sqrt{3}\Rightarrow \theta ={{60}^{o}}\]            and \[m=-\frac{a}{b}\sin \theta =\frac{-4}{3}\sin {{60}^{o}}\] = \[\frac{-4}{3}\times \frac{\sqrt{3}}{2}=\frac{-2}{\sqrt{3}}\].


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