JEE Main & Advanced Mathematics Conic Sections Question Bank Ellipse

  • question_answer
    Eccentricity of the ellipse whose latus rectum is equal to the distance between two focus points, is

    A)            \[\frac{\sqrt{5}+1}{2}\]            

    B)            \[9{{x}^{2}}+5{{y}^{2}}-30y=0\]

    C)            \[\frac{\sqrt{5}}{2}\]                

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

    Correct Answer: B

    Solution :

               \[\frac{2{{b}^{2}}}{a}=2ae\]Þ \[{{b}^{2}}={{a}^{2}}e\]or\[e=\frac{{{b}^{2}}}{{{a}^{2}}}\]            Also \[e=\sqrt{1-\frac{{{b}^{2}}}{{{a}^{2}}}}\]or \[{{e}^{2}}=1-e\]or \[{{e}^{2}}+e-1=0\]            Therefore\[e=\frac{-1\pm \sqrt{5}}{2}\]. As\[e<1,\]\ \[e=\frac{\sqrt{5}-1}{2}\].


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