CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2000

  • question_answer
    If \[{{2}^{x}}+{{2}^{y}}={{2}^{x+y}},\]then the value of \[\frac{dy}{dx}\]at \[x=y=1\]is:

    A)  zero                                     

    B) \[-1\]

    C)  1                                            

    D)  2

    Correct Answer: B

    Solution :

    \[{{2}^{x}}+{{2}^{y}}={{2}^{x+y}}\] Differentiating, we get                  \[{{2}^{x}}\log 2+{{2}^{y}}\log 2\frac{dy}{dx}={{2}^{x+y}}\log 2\left( 1+\frac{dy}{dx} \right)\]                 \[\Rightarrow \]\[{{2}^{x}}+{{2}^{y}}\frac{dy}{dx}={{2}^{x+y}}+{{2}^{x+y}}\frac{dy}{dx}\] \[\Rightarrow \]\[({{2}^{y}}-{{2}^{x+y}})\frac{dy}{dx}={{2}^{x+y}}-{{2}^{x}}\] \[\Rightarrow \]\[\frac{dy}{dx}=\frac{{{2}^{x+y}}-{{2}^{x}}}{{{2}^{y}}-{{2}^{x+y}}}\] \[\Rightarrow \]\[\frac{dy}{dx}=\frac{{{2}^{2}}-2}{2-{{2}^{2}}}=\frac{2}{-2}=-1\]


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