Question #311635

Find a general solution of the following differential equation



dy/dx - 5y = (xe^-2x)*y^-2

1
Expert's answer
2022-03-15T17:26:31-0400

Solve Bernoulli’s equation dy(x)dx5y(x)=e2xxy(x)2:Multiply both sides by 3y(x)2:3dy(x)dxy(x)215y(x)3=3e2xxLet v(x)=y(x)3, which gives dv(x)dx=3y(x)2dy(x)dx:dv(x)dx15v(x)=3e2xx Let μ(x)=e15dx=e15x. Multiply both sides by μ(x):e15xdv(x)dx(15e15x)v(x)=3e17xx Substitute 15e15x=ddx(e15x):e15xdv(x)dx+ddx(e15x)v(x)=3e17xx Apply the reverse product rule fdgdx+gdfdx=ddx(fg) to the left-hand side: ddx(e15xv(x))=3e17xx\text{Solve Bernoulli's equation } \frac{d y(x)}{d x}-5 y(x)=\frac{e^{-2 x} x}{y(x)^{2}} :\\[4mm] \text{Multiply both sides by }3 y(x)^{2} :\\[2mm] 3 \frac{d y(x)}{d x} y(x)^{2}-15 y(x)^{3}=3 e^{-2 x} x\\[4mm] \text{Let } v(x)=y(x)^{3},\text{ which gives } \frac{d v(x)}{d x}=3 y(x)^{2} \frac{d y(x)}{d x}:\\ \frac{d v(x)}{d x}-15 v(x)=3 e^{-2 x} x\\[4mm] \text{ Let } \mu(x)=e^{\int-15 d x}=e^{-15 x}.\\ \text{ Multiply both sides by } \mu(x): \\[2mm] e^{-15 x} \frac{d v(x)}{d x}-\left(15 e^{-15 x}\right) v(x)=3 e^{-17 x} x\\ \text{ Substitute } -15 e^{-15 x}=\frac{d}{d x}\left(e^{-15 x}\right) :\\ e^{-15 x} \frac{d v(x)}{d x}+\frac{d}{d x}\left(e^{-15 x}\right) v(x)=3 e^{-17 x} x\\[4mm] \text{ Apply the reverse product rule } f \frac{d g}{d x}+g \frac{d f}{d x}=\frac{d}{d x}(f g) \text{ to the left-hand side: } \frac{d}{d x}\left(e^{-15 x} v(x)\right)=3 e^{-17 x} x\\


 Evaluate the integrals:e15xv(x)=3e17x(x171289)+c1, where c1 is an arbitrary constant. Divide both sides by μ(x)=e15x:v(x)=1289e2x(51x+289c1e17x3) Solve for y(x) in v(x)=y(x)3:y(x)=e(2x)/351x+289c1e17x33172/3 or y(x)=13e(2x)/351x+289c1e17x33172/3 or y(x)=(1)2/3e(2x)/351x+289c1e17x33172/3 Simplifying the arbitrary constants we have:y(x)=e(2x)/351x+c1e17x33172/3 or y(x)=13e(2x)/351x+c1e17x33172/3 or y(x)=(1)2/3e(2x)/351x+c1e17x33172/3\text{ Evaluate the integrals:}\\[2mm] e^{-15 x} v(x)=3 e^{-17 x}\left(-\frac{x}{17}-\frac{1}{289}\right)+c_{1}, \text{ where } c_{1} \text{ is an arbitrary constant.}\\[3mm] \text{ Divide both sides by } \mu(x)=e^{-15 x} :\\[2mm] v(x)=\frac{1}{289} e^{-2 x}\left(-51 x+289 c_{1} e^{17 x}-3\right)\\[4mm] \text{ Solve for } y(x) \text{ in } v(x)=y(x)^{3}:\\[2mm] \begin{aligned} &y(x)=\frac{e^{-(2 x) / 3} \sqrt[3]{-51 x+289 c_{1} e^{17 x}-3}}{17^{2 / 3}} \\ &\text { or } y(x)=-\frac{\sqrt[3]{-1} e^{-(2 x) / 3} \sqrt[3]{-51 x+289 c_{1} e^{17 x}-3}}{17^{2 / 3}} \\ &\text { or } y(x)=\frac{(-1)^{2 / 3} e^{-(2 x) / 3} \sqrt[3]{-51 x+289 c_{1} e^{17 x}-3}}{17^{2 / 3}} \end{aligned} \\[4mm] \text{ Simplifying the arbitrary constants we have:}\\[4mm] \begin{aligned} &y(x)=\frac{e^{-(2 x) / 3} \sqrt[3]{-51 x+c_{1} e^{17 x}-3}}{17^{2 / 3}} \\ &\text { or } y(x)=-\frac{\sqrt[3]{-1} e^{-(2 x) / 3} \sqrt[3]{-51 x+c_{1} e^{17 x}-3}}{17^{2 / 3}} \\ &\text { or } y(x)=\frac{(-1)^{2 / 3} e^{-(2 x) / 3} \sqrt[3]{-51 x+c_{1} e^{17 x}-3}}{17^{2 / 3}} \end{aligned}


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