JEE Main & Advanced Mathematics Conic Sections Question Bank Parabola

  • question_answer
    The point on the parabola \[{{y}^{2}}=8x\] at which the normal is parallel to the line \[x-2y+5=0\] is

    A)            \[(-1/2,\ 2)\]                             

    B)            \[(1/2,\ -2)\]

    C)            \[(2,\ -1/2)\]                             

    D)            \[(-2,\ 1/2)\]

    Correct Answer: B

    Solution :

               Let point be \[(h,k).\] Normal is \[y-k=\frac{-k}{4}(x-h)\] or \[-kx-4y+kh+4k=0\]            Gradient \[=-\frac{k}{4}=\frac{1}{2}\]Þ \[k=-2\]            Substituting \[(h,k)\] and \[k=-2\], we get \[h=\frac{1}{2}\]            Hence point is \[\left( \frac{1}{2},-2 \right)\].            Trick: Here only point \[\left( \frac{1}{2},-2 \right)\] satisfies the parabola\[{{y}^{2}}=8x\].


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