Find the particular solution of a differential equation

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SUMMARY

The discussion focuses on finding the particular solution of the differential equation dy/dx = (x-4)e^(-2y) with the initial condition y(4)=ln(4). The user successfully separated variables and integrated to obtain e^(2y)/2 = x^2/2 - 4x + C. The confusion arose regarding the constant C and the factor of 1/2 in front of the logarithm during the isolation of y. Participants clarified that C should be determined during integration and emphasized the importance of maintaining the correct form of the equation throughout the solution process.

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TrueStar
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Homework Statement



Find the particular solution of the differential equation dy/dx = (x-4)e^(-2y)

Satisfying initial condition y(4)=ln(4)

Homework Equations



N/A

The Attempt at a Solution



I separated this into dy/e^(-2y) = (x-4)dx

I then integrated it to get e^(2y)/2 = x^2/2 - 4x

I then tried to isolate y and got y= 1/2ln(x^2-8x+C)

Plugging in the values, I get ln(4)=1/2ln(-16+C)

What do I do with that 1/2 out in front? If it wasn't a part of the equation, C would equal 20. That is, if I'm correct so far.
 
Last edited:
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Put the everything inside the ln - it will make you feel happier.

e^{2y} = x^2-8x+c \Rightarrow y=\ln \sqrt{x^2-8x+c}... now the initial condition makes sense?
 
You add the +C after you integrate. Not at the very end. So you should have gotten e^(2y)/2 = x^2/2 - 4x + C. You can actually solve for C at that step as well. Then isolate y.
 
Yeah I realized that, but the 1/2 is still throwing me off. I'm sure it's something simple that I've forgotten.
 
a\ln(x)=ln(x^a)
But solve for c while y is still in the exponential like Dick says also works.
 
TrueStar said:
Yeah I realized that, but the 1/2 is still throwing me off. I'm sure it's something simple that I've forgotten.

I'm not sure what's throwing you off. C isn't 20. It's something else. Solve for it!
 
I forgot the rule that Simon posted. It makes complete sense now. I will look at it the other way as well. Thank you both!
 

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