JEE Main & Advanced Sample Paper JEE Main Sample Paper-37

  • question_answer
    If \[s\], \[s'\] are the length of the perpendicular on a tangent from the foci, \[a,\,\,a'\] are those from the vertices is that from the centre and \[e\] is the eccentricity of the ellipse,\[\frac{{{x}^{2}}}{{{a}^{2}}}+\frac{{{y}^{2}}}{{{b}^{2}}}=1\], then\[\frac{ss'-{{c}^{2}}}{aa'-{{c}^{2}}}=\]

    A) \[e\]                             

    B) \[1/e\]

    C) \[1/{{e}^{2}}\]                                

    D) \[{{e}^{2}}\]

    Correct Answer: D

    Solution :

     Let the equation of tangent \[y=mx+\sqrt{{{a}^{2}}{{m}^{2}}+{{b}^{2}}}\] Foci\[=(\pm ae,\,\,0)\], vertices\[\equiv (\pm a,\,\,0)\equiv (0,\,\,0)\] \[\therefore \]\[s=\left| \frac{mae+\sqrt{{{a}^{2}}{{m}^{2}}+{{b}^{2}}}}{\sqrt{1+{{m}^{2}}}} \right|,\,\,\,s'=\left| \frac{-mae+\sqrt{{{a}^{2}}{{m}^{2}}+{{b}^{2}}}}{\sqrt{1+{{m}^{2}}}} \right|\]\[a=\left| \frac{ma+\sqrt{{{a}^{2}}{{m}^{2}}+{{b}^{2}}}}{\sqrt{1+{{m}^{2}}}} \right|,\,\,a'=\left| \frac{-ma+\sqrt{{{a}^{2}}{{m}^{2}}+{{b}^{2}}}}{\sqrt{1+{{m}^{2}}}} \right|,\]\[c=\left| \frac{\sqrt{{{a}^{2}}{{m}^{2}}+{{b}^{2}}}}{\sqrt{1+{{m}^{2}}}} \right|\] \[\therefore \]    \[\frac{ss'-{{c}^{2}}}{aa'-{{c}^{2}}}=\frac{-\frac{{{m}^{2}}{{a}^{2}}{{e}^{2}}}{1+{{m}^{2}}}}{\frac{{{m}^{2}}{{a}^{2}}}{1+{{m}^{2}}}}={{e}^{2}}\]


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