Help Check DE Homework: x^2y' -2xy = 3y^4

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SUMMARY

The discussion focuses on solving the differential equation x²(dy/dx) - 2xy = 3y⁴ using Bernoulli's equation. The user successfully transforms the equation into standard form and applies u-substitution with w = y^{-3}. The integrating factor is calculated as μ = x⁶, leading to the equation w' + (6/x)w = -9/x². However, the user identifies a discrepancy in the final powers compared to the solution provided by Wolfram Alpha, indicating a potential error in their simplification process.

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Homework Statement



x^{2}\frac{dy}{dx}-2xy=3y^{4}

Homework Equations



Bernoulli's equation

The Attempt at a Solution



x^{2}\frac{dy}{dx}-2xy=3y^{4}

First I divide through by x^{2} and y^{n} to put it in standard form, and then to begin Bernoulli's equation process.

y^{-4}y'-\frac{2}{x}y^{-3}=\frac{3}{x^{2}}

u-substitution:

w=y^{1-n}=y^{-3}
w'=-3y^{-4}y'

Substitute in...
\frac{-1}{3}w'-\frac{2}{x}w=\frac{3}{x^{2}}

Put in Standard Form...

w'+\frac{6}{x}w=-\frac{9}{x^{2}}

Get the integrating factor...

p(x)=\frac{6}{x} \Rightarrow \int\frac{6}{x}dx = 6ln(x)\Rightarrow\mu=e^{6lnx}=x^{6}\int\frac{d}{dx}(w*x^{6})dx=\int\frac{-9}{x^{4}}dx

w=\frac{3}{x^{9}}+cx^{-6}

I'll stop here since I don't think the simplification with help much (w->y^-3)

Wolfram got this:

Clickity

Which kinda-sorta looks like my answer, except the powers are off. Where did I go wrong?

Using wolfram to simplify, with y^-3 plugged in for w
 
Last edited:
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CINA said:

Get the integrating factor...

p(x)=\frac{6}{x} \Rightarrow \int\frac{6}{x}dx = 6ln(x)\Rightarrow\mu=e^{6lnx}=x^{6}


\int\frac{d}{dx}(w*x^{6})dx=\int\frac{-9}{x^{4}}dx



That part is wrong.
 

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