JCECE Engineering JCECE Engineering Solved Paper-2008

  • question_answer
    The equation of tangents to the ellipse\[\frac{{{x}^{2}}}{9}+\frac{{{y}^{2}}}{4}=1\], which are perpendicular to the line \[3x+4y=7\], are

    A) \[4x-3y=\pm \sqrt{20}\]              

    B) \[4x-3y=\pm \sqrt{12}\]

    C) \[4x-3y=\pm \sqrt{2}\] 

    D) \[4x-3y=\pm 1\]

    Correct Answer: A

    Solution :

    Let the equation of tangent, which is perpendicular to the line\[3x+4y=7\], is\[4x-3y=\lambda \]. Since, it is a tangent to the ellipse. \[\therefore \]  \[{{\lambda }^{2}}={{a}^{2}}{{m}^{2}}+{{b}^{2}}\] Here,\[{{a}^{2}}=9,\,\,{{b}^{2}}=4\]and\[m=\frac{4}{3}\] \[\therefore \]                  \[{{\lambda }^{2}}=9\times {{\left( \frac{4}{3} \right)}^{2}}+4\]                                 \[=16+4\] \[\Rightarrow \]                               \[{{\lambda }^{2}}=20\] \[\Rightarrow \]                               \[\lambda =\pm \sqrt{20}\] \[\therefore \]Equation is\[4x-3y=\pm \sqrt{20}\]


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