RAJASTHAN ­ PET Rajasthan PET Solved Paper-2009

  • question_answer
    If \[{{x}^{y}}={{e}^{x-y}},x>0,\] then value of \[\frac{dy}{dx}\] at (1,1) is

    A)  0

    B)   \[\frac{1}{2}\]

    C)  1

    D)  2

    Correct Answer: A

    Solution :

     Given, \[{{x}^{y}}={{e}^{x-y}}\] Taking log on both sides, we get \[y\text{ }log\text{ }x=x-y\] \[\Rightarrow\] \[\frac{y}{x}+\log x\frac{dy}{dx}=1-\frac{dy}{dx}\] \[\Rightarrow\] \[\frac{dy}{dx}(1+\log x)=1-\frac{y}{x}\] \[\Rightarrow\] \[{{\left( \frac{dy}{dx} \right)}_{(1,1)}}(1+\log 1)=1-\frac{1}{1}\] \[\Rightarrow\] \[{{\left( \frac{dy}{dx} \right)}_{(1,1)}}=0\]


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