JCECE Engineering JCECE Engineering Solved Paper-2015

  • question_answer
    The subtangent at any point on the curve \[{{x}^{m}}{{y}^{n}}={{a}^{m+n}}\]varies as

    A) \[{{(abscissa)}^{2}}\]

    B) \[{{(ordinate)}^{2}}\]

    C) \[abscissa\]

    D) \[ordinate\]

    Correct Answer: C

    Solution :

    We have,                 \[{{x}^{m}}{{y}^{n}}={{a}^{m+n}}\] \[\Rightarrow \]\[m{{\log }_{e}}x+n{{\log }_{e}}y=(m+n){{\log }_{e}}a\] On differentiating both sides w.r.t. x, we get \[\Rightarrow \]               \[\frac{m}{x}+\frac{n}{y}\cdot \frac{dy}{dx}=0\] \[\Rightarrow \]               \[\frac{dy}{dx}=-\frac{my}{mx}\] \[\therefore \]Length of the subtangent\[=\left| \frac{y}{dy/dx} \right|\]                                                 \[=\left| -y\times \frac{nx}{my} \right|\]                                                 \[=\frac{n}{m}|x|\propto x\]


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