JEE Main & Advanced Mathematics Conic Sections Question Bank Ellipse

  • question_answer
    The equation of ellipse whose distance between the foci is equal to 8 and distance between the directrix is 18, is

    A)            \[5{{x}^{2}}-9{{y}^{2}}=180\]   

    B)            \[9{{x}^{2}}+5{{y}^{2}}=180\]

    C)            \[{{x}^{2}}+9{{y}^{2}}=180\]     

    D)            \[5{{x}^{2}}+9{{y}^{2}}=180\]

    Correct Answer: D

    Solution :

               \[2ae=8,\,\,\frac{2a}{e}=18\]Þ \[a=\sqrt{4\times 9}=6\]                    \[e=\frac{2}{3},\]\[b=6\sqrt{1-\frac{4}{9}}=\frac{6}{3}\sqrt{5}=2\sqrt{5}\]                    Hence the required equation is \[\frac{{{x}^{2}}}{36}+\frac{{{y}^{2}}}{20}=1\]                    i.e., \[5{{x}^{2}}+9{{y}^{2}}=180\].


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