JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2011

  • question_answer
        A point on the parabola\[{{y}^{2}}=18x\]at which the ordinate increases at twice the rate of the abscissa is

    A)  \[\left( -\frac{9}{8},\frac{9}{2} \right)\]                

    B)  \[(2,-4)\]

    C)  \[(2,4)\]                             

    D)  \[\left( \frac{9}{8},\frac{9}{2} \right)\]

    Correct Answer: D

    Solution :

                    Given curve is\[{{y}^{2}}=18x\] \[\therefore \]  \[2y\frac{dy}{dt}=18\frac{dx}{dt}\] \[\Rightarrow \] \[2y\left( 2\frac{dx}{dt} \right)=18\frac{dx}{dt}\]            \[\left( \because \frac{dy}{dt}=2\frac{dx}{dt}given \right)\] \[\Rightarrow \]               \[y=\frac{9}{2}\] \[\therefore \]  \[x=\frac{{{y}^{2}}}{18}=\frac{81}{4\times 18}=\frac{9}{8}\]


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