JEE Main & Advanced JEE Main Paper (Held on 10-4-2019 Morning)

  • question_answer
    If a directrix of a hyperbola centred at the origin and passing through the point \[\left( 4,-2\sqrt{3} \right)\]is \[5x=4\sqrt{5}\]and its eccentricity is e, then : [JEE Main 10-4-2019 Morning]

    A) \[4{{e}^{4}}-24{{e}^{2}}+35=0\]        

    B) \[4{{e}^{4}}+8{{e}^{2}}-35=0\]

    C) \[4{{e}^{4}}-12{{e}^{2}}-27=0\]

    D)   \[4{{e}^{4}}-24{{e}^{2}}+27=0\]

    Correct Answer: B

    Solution :

    Hyperbola is\[\frac{{{x}^{2}}}{{{a}^{2}}}-\frac{{{y}^{2}}}{{{b}^{2}}}=1\]             \[\frac{a}{e}=\frac{4}{\sqrt{5}}\]and\[\frac{16}{{{a}^{2}}}-\frac{12}{{{b}^{2}}}=1\]             \[{{a}^{2}}=\frac{16}{5}{{e}^{2}}\]                                        ?.(1)             and\[\frac{16}{{{a}^{2}}}-\frac{12}{{{a}^{2}}({{e}^{2}}-1)}=1\]                        ?(2) From (1) & (2) \[16\left( \frac{5}{16{{e}^{2}}} \right)-\frac{12}{({{e}^{2}}-1)}\left( \frac{5}{16{{e}^{2}}} \right)=1\]\[16\left( \frac{5}{16{{e}^{2}}} \right)-\frac{12}{({{e}^{2}}-1)}\left( \frac{5}{16{{e}^{2}}} \right)=1\] \[\Rightarrow 4{{e}^{4}}-24{{e}^{2}}+35=0\]


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