y'+e^{y'}-x=0 that I have simplified like so e^{y'}=x-y' \ln e^{y'}=\ln(x-y') y'=\ln(x-y') but I do not

dristauwaz

dristauwaz

Answered question

2022-02-16

y+eyx=0
that I have simplified like so
ey=xy
lney=ln(xy)
y=ln(xy)
but I do not know how to solve this further to obtain the general solution. I have done first order linear differential equation strategies so far. How should I get about doing this question with the strategies I have?

Answer & Explanation

Nicolle Newman

Nicolle Newman

Beginner2022-02-17Added 4 answers

Your (revised) simplification doesn't really simplify because it separates the ys from each other. The productive thing to do would be to rewrite it to
x=y+ey
and work from there.
First, let g(u)=u+eu; then the equation becomes x=g(y) or y=g1(x). In this form we get the solution
y=1xg1(t)dt+C
This integral is not nice, but (noting that g1(1)=0, which is why I chose 1 as the lower limit) we can flip it around to
y=xg1(x)0g1(x)g(u)du+C
and g is easy enough to integrate so we get
y=xg1(x)g1(x)22eg1(x)+C1
(with C1=C=1). With a bit of help from Wolfram Alpha we find g1(x)=xW(ex) where W is the Lambert W function. We can plug this in to get
y=x22W(ex)22+exW(ex)+C2
By the definition of W we have eW(ex)=W(ex)ex, so this further simplifies to
y=x2(W(ex)1)22+C3
If still not exactly pretty, this is at least a "closed form" modulo the W function.
homofilirix

homofilirix

Beginner2022-02-18Added 8 answers

Thanks for your answer.

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