KVPY Sample Paper KVPY Stream-SX Model Paper-1

  • question_answer
    A hyperbola, having the transverse axis of length \[2\,\sin \theta \] is confocal with the ellipse \[3{{x}^{2}}+4{{y}^{2}}=12.\] Then, its equation is

    A) \[{{x}^{2}}\text{cose}{{\text{c}}^{2}}\theta -{{y}^{2}}{{\sec }^{2}}\theta =1\]

    B) \[{{x}^{2}}{{\sec }^{2}}\theta -{{y}^{2}}\text{cose}{{\text{c}}^{2}}\theta =1\]

    C) \[{{x}^{2}}{{\sin }^{2}}\theta -{{y}^{2}}{{\cos }^{2}}\theta =1\]

    D) \[{{x}^{2}}{{\cos }^{2}}\theta -{{y}^{2}}{{\sin }^{2}}\theta =1\]

    Correct Answer: A

    Solution :

    [a]
    Equation of ellipse
    \[3{{x}^{2}}+4{{y}^{2}}=12\]
    \[\Rightarrow \]\[\frac{{{x}^{2}}}{4}+\frac{{{y}^{2}}}{3}=1\]
    Focus of ellipse \[(\pm 1,0).\]
    Focus of ellipse is also focus of hyperbola.
    \[\therefore \]      \[ae=1\]
    Length of transverse axis \[(2a)=2\sin \theta \]
    \[\therefore \]      \[a=\sin \theta \]
    In hyperbola \[{{(ae)}^{2}}={{a}^{2}}+{{b}^{2}}\]
    \[1={{\sin }^{2}}\theta +{{b}^{2}}\]
    \[\Rightarrow \]   \[{{b}^{2}}={{\cos }^{2}}\theta \]
    Equation of hyperbola \[\frac{{{x}^{2}}}{{{\sin }^{2}}\theta }-\frac{{{y}^{2}}}{{{\cos }^{2}}\theta }=1\]
    \[\Rightarrow \]\[{{x}^{2}}\text{cose}{{\text{c}}^{2}}\theta -{{y}^{2}}{{\sec }^{2}}\theta =1\]


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