JEE Main & Advanced Mathematics Differential Equations Question Bank Exact defferential equations

  • question_answer
    The solution of \[(1+xy)y\,dx+(1-xy)x\,dy=0\] is

    A)                 \[\frac{x}{y}+\frac{1}{xy}=k\]         

    B)                 \[\log \left( \frac{x}{y} \right)=\frac{1}{xy}+k\]

    C)                 \[\frac{x}{y}+\frac{1}{xy}=k\]         

    D)                 \[\log \left( \frac{x}{y} \right)=xy+k\]

    Correct Answer: B

    Solution :

                       \[ydx+xdy+x{{y}^{2}}dx-{{x}^{2}}ydy=0\]                    \[\frac{ydx+xdy}{{{x}^{2}}{{y}^{2}}}+\frac{dx}{x}-\frac{dy}{y}=0\].  On integrating, we get                                 \[-\frac{1}{xy}+\log x-\log y=k\] Þ \[\log \frac{x}{y}=\frac{1}{xy}+k\].


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