Are both results valid solutions for the separable DE? I have a separable differential equation (with no initial value) dx + (2z-1)/(2-z) dz = 0

erwachsenc6

erwachsenc6

Answered question

2022-10-23

Are both results valid solutions for the separable DE?
I have a separable differential equation (with no initial value)
d x + 2 z 1 2 z d z = 0
And the way my professor did it is
d x = x + C
2 z 1 2 z d z = ( 2 ( z 2 ) z 2 + 3 z 2 ) d z
= 2 z 3 l n | z 2 |
Resulting in
x 2 z 3 l n | z 2 | = C 1
The way that he did it is with both integrals on the same side.
What I did was first moved them to separate sides, with the signs changing as necessary.
d x = 1 2 z 2 z d z
1 2 z 2 z = 2 ( 2 z ) 3 2 z
So then my integral becomes
1 2 z 2 z d z = 2 2 z 2 z d z 3 1 2 z d z
The result being
2 z 3 l n | 2 z |
Rearranging to get the final solution I end up with
x 2 z + 3 l n | 2 z | = C 1
The two solutions differ because of the manipulation earlier from changing sides and moving them back afterwards.
Professor's solution:
x 2 z 3 l n | z 2 | = C 1
My solution:
x 2 z + 3 l n | 2 z | = C 1
My question is, are both of these valid solutions? The domain where ln is defined is different in both solutions. If there is a mistake, what have I done wrong?
Thank you.

Answer & Explanation

Jimena Torres

Jimena Torres

Beginner2022-10-24Added 20 answers

You need to carry out the implicit substitution u = 2 z, d u = d z correctly,
d z 2 z = d ( 2 z ) 2 z = ln | 2 z | + C .

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