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

  • question_answer
    The solution of \[(x-{{y}^{3}})dx+3x{{y}^{2}}dy=0\] is

    A)                 \[\log x+\frac{x}{y}\]           

    B)                 \[\log x+\frac{{{y}^{3}}}{x}=k\]

    C)                 \[\log x-\frac{x}{{{y}^{3}}}=k\]     

    D)                 \[\log xy-{{y}^{3}}=k\]

    Correct Answer: B

    Solution :

                       \[xdx-{{y}^{3}}dx+3x{{y}^{2}}dy=0\]                    Put \[{{y}^{3}}=t\] \[\Rightarrow \] \[dt=3{{y}^{2}}dy\]                    \[x\,dx-tdx+xdt=0\] Þ \[xdx+xdt-tdx=0\]                    Þ \[\frac{dx}{x}+d\left( \frac{t}{x} \right)=0\]                                 On integration, we get \[\log x+\frac{t}{x}=k\] Þ \[\log x+\frac{{{y}^{3}}}{x}=k\].             


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