JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Application in Mechanics and Rate Measurer

  • question_answer
    A point on the parabola \[{{y}^{2}}=18x\]at which the ordinate increases at twice the rate of the abscissa is  [AIEEE 2004]

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

    B)            (2, ? 4)

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

    D)            (2, 4)

    Correct Answer: A

    Solution :

                \[{{y}^{2}}=18x\]               Differentiate both sides w.r.t. t                    \[2y\left( \frac{dy}{dt} \right)=18\left( \frac{dx}{dt} \right)\]                     Þ \[2y\left( 2\frac{dx}{dt} \right)=18\left( \frac{dx}{dt} \right)\],\[\left( \because \frac{dy}{dt}=2\frac{dx}{dt} \right)\]                    \ \[4y=18\] or \[y=\frac{9}{2}\] and \[x=\frac{{{y}^{2}}}{18}=\frac{9}{8}\]                    Hence the required point is \[\left( \frac{9}{8},\frac{9}{2} \right)\].


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