J & K CET Engineering J and K - CET Engineering Solved Paper-2008

  • question_answer
    One of the directrices of the ellipse \[8{{x}^{2}}+6{{y}^{2}}-16x+12y+13=0\]is

    A)  \[3y-3=\sqrt{6}\]  

    B)  \[3y+3=\sqrt{6}\]

    C)  \[y+1=\sqrt{3}\]    

    D)  \[y-1=-\sqrt{3}\]

    Correct Answer: B

    Solution :

    Given that, \[8({{x}^{2}}-2x+1)+6({{y}^{2}}+2y+1)\] \[+13-8-6=0\] \[\Rightarrow \] \[\frac{{{(x-1)}^{2}}}{1/8}+\frac{{{(y+1)}^{2}}}{1/6}=1\] Here, \[b>a\] Now, \[e=\sqrt{1-\frac{1/8}{1/6}}=\sqrt{\frac{2}{8}}=\frac{1}{2}\] \[\therefore \]  Directrix,  \[y+1=\pm \left( \frac{\sqrt{1/6}}{1/2} \right)\,\,\,\left[ \because \,\,y=\pm \frac{b}{e} \right]\] \[\Rightarrow \] \[y+1=\pm \frac{2}{\sqrt{6}}\] \[\Rightarrow \] \[3y+3=\pm \sqrt{6}\]


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