CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2003

  • question_answer
    For the curve \[xy={{c}^{2}}\] the subnormal at any point varies as :

    A)  \[{{x}^{3}}\]                                     

    B)  \[{{x}^{2}}\]

    C)  \[{{y}^{3}}\]                                     

    D)  \[\infty \]

    Correct Answer: C

    Solution :

    We have \[xy={{c}^{2}}\] \[\therefore \]  \[y=\frac{{{c}^{2}}}{x}\]                                                                ... (i) \[\therefore \]  \[\frac{dy}{dx}={{c}^{2}}\left( -\frac{1}{{{x}^{2}}} \right)\] \[\therefore \] Sub normal at any point \[=y\,.\,\frac{dy}{dx}\]                 \[=y\times \left( -\frac{{{c}^{2}}}{{{x}^{2}}} \right)\]                 \[=y\times \frac{-{{c}^{2}}}{{{\left( \frac{{{c}^{2}}}{y} \right)}^{2}}}\]                      \[\left[ x=\frac{{{c}^{2}}}{y} \right]\]                 \[=y\times \frac{-{{c}^{2}}{{y}^{2}}}{{{c}^{4}}}\]                 \[=-\frac{{{y}^{3}}}{{{c}^{2}}}\] \[\therefore \] Subnormal \[\propto {{y}^{3}}\]


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