KVPY Sample Paper KVPY Stream-SX Model Paper-17

  • question_answer
    Consider a circle with its centre lying on the focus of the parabola \[{{y}^{2}}\text{ }=\text{ }2px\] such that it touches the directrix of the parabola. Then a point of intersection of the circle and the parabola is

    A) \[\left( \frac{p}{2},p \right)\]

    B) \[(2p,2p)\]

    C) \[\left( -\frac{p}{2},p \right)\]

    D) \[\left( -\frac{p}{8},\frac{p}{2} \right)\]

    Correct Answer: A

    Solution :

    The focus of the parabola is at \[\left( \frac{p}{2},0 \right)\] and directrix is \[x=-\frac{p}{2}\]
    Centre of the circle is \[\left( \frac{p}{2},0 \right)\] and radius \[=\frac{p}{2}-\left( -\frac{p}{2} \right)=p\]
    Equation of circle is \[{{\left( x-\frac{p}{2} \right)}^{2}}+{{\left( y-0 \right)}^{2}}={{p}^{2}}\]\[\Rightarrow \]\[{{x}^{2}}+{{y}^{2}}-px-\frac{3{{p}^{2}}}{4}=0\]
    solving this equation with \[{{y}^{2}}=2px,\] we obtain \[x=\frac{p}{2}\]and\[y=\pm p\]
    \[\therefore \]the points of intersection of the circle and the parabola are \[\left( \frac{p}{2},p \right)\] and \[\left( \frac{p}{2},-p \right).\]


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