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

  • question_answer
    The solution of the equation \[(x+2{{y}^{3}})\frac{dy}{dx}-y=0\] is [MP PET 1998; 2002]

    A)                 \[y(1-xy)=Ax\]      

    B)                 \[{{y}^{3}}-x=Ay\]

    C)                 \[x(1-xy)=Ay\]      

    D)                 \[x(1+xy)=Ay\] Where A is any arbitrary constant

    Correct Answer: B

    Solution :

                       \[(x+2{{y}^{3}})\frac{dy}{dx}=y\] Þ \[\frac{dy}{dx}=\frac{y}{x+2{{y}^{3}}}\]                    Þ \[\frac{dx}{dy}=\frac{x+2{{y}^{3}}}{y}\] or \[\frac{dx}{dy}-\frac{x}{y}=2{{y}^{2}}\], which is a linear equation of the form \[\frac{dx}{dy}+Px=Q\]                    So, integrating factor (I.F.)\[={{e}^{-\int_{{}}^{{}}{\frac{1}{y}dy}}}\]and solution is \[x\frac{1}{y}=\int_{{}}^{{}}{\frac{1}{y}2{{y}^{2}}dy+A={{y}^{2}}+A}\] Þ \[x={{y}^{3}}+Ay\]                                 Þ \[{{y}^{3}}-x=Ay;\]where A can be \[-ve\]or \[+ve\].


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