J & K CET Engineering J and K - CET Engineering Solved Paper-2011

  • question_answer
    If the straight line \[y=2x+c\]is a tangent to the ellipse \[\frac{{{x}^{2}}}{3}+\frac{{{y}^{2}}}{4}=1,\] then c equals to

    A)  \[\pm \,\,4\]

    B)  \[\pm \,\,6\]

    C)  \[\pm \,\,8\]               

    D)  \[\pm \,\,1\]

    Correct Answer: A

    Solution :

    Given that, the straight line \[y=2x+c\] is the tangent to the ellipse \[\frac{{{x}^{2}}}{3}+\frac{{{y}^{2}}}{4}=1.\] Then by condition, \[c=\sqrt{{{a}^{2}}{{m}^{2}}+{{b}^{2}}}\] \[\Rightarrow \] \[c=\sqrt{(3){{(2)}^{2}}+(4)}\] \[\left( \because \,\,\,\left\{ \begin{matrix}    m=2  \\    {{a}^{2}}=3  \\    {{b}^{2}}=4  \\ \end{matrix} \right. \right)\] \[\Rightarrow \] \[c=\sqrt{12+4}=\sqrt{16}\] \[c=\pm 4\]


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