JEE Main & Advanced JEE Main Paper Phase-I Held on 07-1-2020 Morning

  • question_answer
    If the distance between the foci of an ellipse is 6 and the distance between its directrices is 12, then the length of its latus rectum is   [JEE MAIN Held on 07-01-2020 Morning]

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

    B) \[\sqrt{3}\]

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

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

    Correct Answer: C

    Solution :

    [c] \[2ae=6\] \[\frac{2a}{e}=12\] Multiplying both; \[4{{a}^{2}}=72\] \[{{a}^{2}}=18\] \[\Rightarrow e=\frac{6}{2.3\sqrt{2}}=\frac{1}{\sqrt{2}}\] \[{{e}^{2}}=1-\frac{{{b}^{2}}}{{{a}^{2}}}=\frac{1}{2}\] \[\frac{18}{2}={{b}^{2}}\Rightarrow {{b}^{2}}=9\] \[\therefore \] Latus rectum \[=\frac{2{{b}^{2}}}{a}=\frac{2\times 9}{3\sqrt{2}}=\frac{6}{\sqrt{2}}=3\sqrt{2}\]


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