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

  • question_answer
    Solution of the equation \[(x+\log y)dy+y\,dx=0\] is

    A)                 \[xy+y\log y=c\]   

    B)                 \[xy+y\log y-y=c\]

    C)                 \[xy+\log y-x=c\] 

    D)                 None of these

    Correct Answer: B

    Solution :

                       \[xdy+ydx+\log ydy=0\] Þ \[xdy+ydx=-\log ydy\]                    \[y\frac{dx}{dy}+x=-\log y\] Þ \[\frac{dx}{dy}+\frac{x}{y}=-\frac{\log y}{y}\]                    I.F. =\[{{e}^{\int{\frac{1}{y}dy}}}=y\]                    Hence solution is \[x.y=-\int{y.\frac{\log ydy}{y}+c}\]                                 Þ \[xy=-(y\log y-y)+c\] Þ \[xy+(y\log y-y)=c\].


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