JEE Main & Advanced Sample Paper JEE Main - Mock Test - 17

  • question_answer
    The eccentricity of the hyperbola \[\frac{{{x}^{2}}}{{{a}^{2}}}-\frac{{{y}^{2}}}{{{b}^{2}}}=1\] is reciprocal to that of the ellipse\[{{x}^{2}}+4{{y}^{2}}=4.\]. If the hyperbola passes through a focus of the ellipse, then

    A) The equation of the hyperbola is \[\frac{{{x}^{2}}}{3}-\frac{{{y}^{2}}}{2}=1\]    

    B) A focus of the hyperbola is \[(\sqrt{3},0)\]

    C) The eccentricity of the hyperbola is \[\sqrt{\frac{5}{3}}\]              

    D) The equation of the hyperbola is \[{{x}^{2}}-3{{y}^{2}}=3\]

    Correct Answer: D

    Solution :

    [d] Given ellipse is  \[\frac{{{x}^{4}}}{4}+\frac{{{y}^{2}}}{1}=1.\] Eccentricity of the ellipse \[=\sqrt{1-\frac{1}{4}}=\frac{\sqrt{3}}{2}\] Focus of the ellipse \[=(-\sqrt{3},\,0)\] Eccentricity of the hyperbola \[=\sqrt{1+\frac{{{b}^{2}}}{{{a}^{2}}}}\] So,       \[\sqrt{1+\frac{{{b}^{2}}}{{{a}^{2}}}}=\frac{2}{\sqrt{3}}\]      \[\Rightarrow \,\,\frac{b}{a}=\frac{1}{\sqrt{3}}\] Since the hyperbola passes through the focus of the ellipse, \[\frac{3}{{{a}^{2}}}-0=1\] \[\Rightarrow \,\,\,\,{{a}^{2}}=3\] and \[{{b}^{2}}=1\] Thus, the equation of the hyperbola is \[\frac{{{x}^{2}}}{3}-\frac{{{y}^{2}}}{1}=1\]   or   \[{{x}^{2}}-3{{y}^{2}}=3.\] Focus of hyperbola is \[(\pm \,2,\,0)\].


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