JEE Main & Advanced Mathematics Differential Equations Question Bank Mock Test - Differential Equations

  • question_answer
    If \[y=\frac{x}{\log \left| cx \right|}\] (where c is an arbitrary constant) I the general solution of the differential equation \[dy/dx=y/x+\phi (x/y)\] then the function \[\phi (x/y)\]is

    A) \[{{x}^{2}}/{{y}^{2}}\]        

    B) \[-{{x}^{2}}/{{y}^{2}}\]

    C) \[{{y}^{2}}/{{x}^{2}}\]        

    D) \[-{{y}^{2}}/{{x}^{2}}\]

    Correct Answer: D

    Solution :

    [d] \[\log c+\log \left| x \right|=\frac{x}{y}\] Differentiating w.r.t. x, \[\frac{1}{x}=\frac{y-x\frac{dy}{dx}}{{{y}^{2}}}\] Or \[\frac{{{y}^{2}}}{x}=y-x\frac{dy}{dx}\] Or \[\frac{dy}{dx}=\frac{y}{x}-\frac{{{y}^{2}}}{{{x}^{2}}}\] Or \[\phi \left( \frac{x}{y} \right)=-\frac{{{y}^{2}}}{{{x}^{2}}}\]


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