JEE Main & Advanced JEE Main Paper (Held On 12-Jan-2019 Morning)

  • question_answer
    Let \[y=y(x)\]be the solution of the differential equation, \[x\frac{dy}{dx}+y=x{{\log }_{e}}x,\]\[(x>1)\]. If \[2y(2)=lo{{g}_{e}}4-1,\]then\[y(e)\]is equal to [JEE Main Online Paper Held On 12-Jan-2019 Morning]

    A) \[\frac{{{e}^{2}}}{4}\]                                               

    B) \[-\frac{{{e}^{2}}}{2}\]

    C) \[-\frac{e}{2}\]                          

    D)   \[\frac{e}{4}\]

    Correct Answer: D

    Solution :

    Here, \[\frac{dy}{dx}+\frac{y}{x}={{\log }_{e}}x\] \[\therefore \]\[I.F.={{e}^{\int_{{}}^{{}}{\frac{1}{x}}dx}}=x\] The required solution is \[xy=\int_{{}}^{{}}{{{\log }_{e}}}x.xdx+C\] \[\Rightarrow \]\[xy=\frac{{{x}^{2}}}{2}{{\log }_{e}}x-\frac{{{x}^{2}}}{4}+C\]                     ?(i) \[\because \]\[2y(2)=lo{{g}_{e}}4-1\Rightarrow 2y(2)=2lo{{g}_{e}}2-1\] From (i), C = 0 So,\[y=\frac{x}{2}{{\log }_{e}}x-\frac{x}{4}\Rightarrow y(e)=\frac{e}{2}-\frac{e}{4}=\frac{e}{4}\]


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