VIT Engineering VIT Engineering Solved Paper-2011

  • question_answer
    The locus of the mid-points of the focal chord of the parabola\[{{y}^{2}}=4ax\]is

    A)  \[{{y}^{2}}=a\,(x-a)\]

    B)  \[{{y}^{2}}=2a\,(x-a)\]

    C)  \[{{y}^{2}}=4a\,(x-a)\]

    D)  None of these

    Correct Answer: B

    Solution :

    Any chord PQ which bisected point \[R(h,\,k)\]is \[T=S\]or i.e., \[ky-2a(x+h)={{k}^{2}}-4ah\] Since, it is a focal chord, so it must passes through focus \[(a,\,0)\]. \[\therefore \]   \[k(0)-2a(a+h)={{k}^{2}}-4ah\] \[\Rightarrow \] \[{{k}^{2}}=2ah-4{{a}^{2}}\] Hence, locus is \[{{y}^{2}}=2a(x-a)\]


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