JEE Main & Advanced JEE Main Paper (Held On 26 May 2012)

  • question_answer
    The normal at  \[\left( 2,\frac{3}{2} \right)\]to the ellipse,\[\frac{{{x}^{2}}}{16}+\frac{{{y}^{2}}}{3}=1\] touches a parabola, whose equation is.   JEE Main Online Paper (Held On 26-May-2012)  

    A) \[{{y}^{2}}=-104x\]                         

    B)                        \[{{y}^{2}}=14x\]

    C)                        \[{{y}^{2}}=26x\]                             

    D)                        \[{{y}^{2}}=-14x\]

    Correct Answer: A

    Solution :

                    Ellipse is\[\frac{{{x}^{2}}}{16}+\frac{{{y}^{2}}}{3}=1\] Now, equation of normal at (2, 3/2) is \[\frac{16x}{2}-\frac{3y}{3/2}=16-3\]\[\Rightarrow \]\[8x-2y=13\] \[\Rightarrow \]\[y=4x-\frac{13}{2}\] Let \[y=4x-\frac{13}{2}\]touches a parabola\[{{y}^{2}}=4ax.\] We know, a straight \[y=mx+c\] touchesa parabola \[{{y}^{2}}=4ax\] if\[a-mc=0\] \[\therefore \]\[a-\left( 4 \right)\left( -\frac{13}{2} \right)=0\Rightarrow a=-26\] Hence, required equation of parabola is \[{{y}^{2}}=4(-26)x=-104x\]


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