JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Tangent and Normal

  • question_answer
    For the curve \[xy={{c}^{2}}\] the subnormal at any point varies as                                                               [Karnataka CET 2003]

    A)            \[{{x}^{2}}\]

    B)            \[{{x}^{3}}\]

    C)            \[{{y}^{2}}\]

    D)            \[{{y}^{3}}\]

    Correct Answer: D

    Solution :

               \[xy={{c}^{2}}\]                                                     ?..(i)            \[\because \] Subnormal = \[y\frac{dy}{dx}\]                    \ From (i), \[y=\frac{{{c}^{2}}}{x}\] Þ \[\frac{dy}{dx}=\frac{-{{c}^{2}}}{{{x}^{2}}}\]            \Subnormal\[=\frac{y\times (-{{c}^{2}})}{{{x}^{2}}}=\frac{-y{{c}^{2}}}{{{\left( \frac{{{c}^{2}}}{y} \right)}^{2}}}=\frac{-y{{c}^{2}}{{y}^{2}}}{{{c}^{4}}}=\frac{-{{y}^{3}}}{{{c}^{2}}}\]            \ Subnormal varies as \[{{y}^{3}}.\]


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