Question #297348

{F} Y(4x+y)dx-2(x^2-y)dy=0

1
Expert's answer
2022-02-16T17:36:11-0500

Solution:

(2y2x2)dy+(y2+4xy)dx=0\left(2 y-2 x^{2}\right) d y+\left(y^{2}+4 x y\right) d x=0

Equation in total differentials μ(x,y)M(x,y)dy+μ(x,y)N(x,y)dx=0\mu(x, y) M(x, y) \mathrm{d} y+\mu(x, y) N(x, y) \mathrm{d} x=0  

where M(x,y)=2y2x2 and N(x,y)=y2+4xyM(x, y)=2 y-2 x^{2}\ and\ N(x, y)=y^{2}+4 x y

Check for full differential: M(x,y)x=4x2y+4x=N(x,y)yM(x, y)_{x}^{\prime}=-4 x \neq 2 y+4 x=N(x, y)_{y}^{\prime}  

Search for an integrating factor μ(x,y)?M(x,y)x=Mx and N(x,y)y=Ny\mu(x, y)^{?} M(x, y)_{x}^{\prime}=\frac{\partial M}{\partial x}\ and\ N(x, y)_{y}^{\prime}=\frac{\partial N}{\partial y}  From condition: MμxNμy=μ(NyMx)M \frac{\partial \mu}{\partial x}-N \frac{\partial \mu}{\partial y}=\mu\left(\frac{\partial N}{\partial y}-\frac{\partial M}{\partial x}\right)

Let μ(x,y)=μ(y)μx=0\mu(x, y)=\mu(y) \Rightarrow \frac{\partial \mu}{\partial x}=0  then the condition becomes: 1μdμdy=1N(NyMx)\frac{1}{\mu} \frac{\mathrm{d} \mu}{\mathrm{d} y}=-\frac{1}{N}\left(\frac{\partial N}{\partial y}-\frac{\partial M}{\partial x}\right)  where right side - function from y

dμμ=1N(NyMx)dy=2y dy=2ln(y)ln(μ)=2ln(y)μ=1y2\begin{gathered} \int \frac{\mathrm{d} \mu}{\mu}=\int-\frac{1}{N}\left(\frac{\partial N}{\partial y}-\frac{\partial M}{\partial x}\right) \mathrm{d} y=\int-\frac{2}{y} \mathrm{~d} y=-2 \ln (y) \\ \ln (\mu)=-2 \ln (y) \Rightarrow \mu=\frac{1}{y^{2}} \end{gathered}

Multiply the differential equation by 1y2\frac{1}{y^{2}}

(2y2x2y2)dy+(4xy+1)dx=0,\left(\frac{2}{y}-\frac{2 x^{2}}{y^{2}}\right) \mathrm{d} y+\left(\frac{4 x}{y}+1\right) \mathrm{d} x=0, \quad  lost when dividing solution: 1y2y=0\frac{1}{y^{2}} \rightarrow y=0

Equation in total differentials M(x,y)dy+N(x,y)dx=0M(x, y) \mathrm{d} y+N(x, y) \mathrm{d} x=0  

where M(x,y)=2y2x2y2 and N(x,y)=4xy+1M(x, y)=\frac{2}{y}-\frac{2 x^{2}}{y^{2}}\ and\ N(x, y)=\frac{4 x}{y}+1

Check for full differential: M(x,y)x=N(x,y)y=4xy2M(x, y)_{x}^{\prime}=N(x, y)_{y}^{\prime}=-\frac{4 x}{y^{2}}

Find F(x,y):dF(x,y)=Fydy+FxdxF(x, y): \mathrm{d} F(x, y)={F}_{y}^{\prime} \mathrm{d} y+F_{x}^{\prime} \mathrm{d} x

F(x,y)=N(x,y)dx=y4xy+1 dx=2x2y+x+Cy(2x2y+x)y=2x2y2Cy=M(x,y)(2x2y+x)ydy=2y dy=2ln(y)F(x,y)=2x2y+x+Cy=2ln(y)+2x2y+x\begin{gathered} F(x, y)=\int N(x, y) \mathrm{d} x=\int_{y}^{\prime} \frac{4 x}{y}+1 \mathrm{~d} x=\frac{2 x^{2}}{y}+x+C_{y} \\ \left(\frac{2 x^{2}}{y}+x\right)_{y }^{\prime}=-\frac{2 x^{2}}{y^{2}} \\ C_{y}=\int M(x, y)-\left(\frac{2 x^{2}}{y}+x\right)_{y}^{\prime} \mathrm{d} y=\int \frac{2}{y} \mathrm{~d} y=2 \ln (y) \\ F(x, y)=\frac{2 x^{2}}{y}+x+C_{y}=2 \ln (y)+\frac{2 x^{2}}{y}+x \end{gathered}

Thus, the solution is 2ln(y)+2x2y+x=C2 \ln (y)+\frac{2 x^{2}}{y}+x=C


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