I have the following differential equation s ( 1 &#x2212;<!-- − --> s ) t

bandikizaui

bandikizaui

Answered question

2022-07-02

I have the following differential equation
s ( 1 s ) t = f ( t ) ( f ( t ) s t )
Initial condition: f ( 0 ) = 0.
I solved it and got the solution as
f ( t ) = 2 c 1 + s 2 2 ( s 1 ) s ln ( t ) + s
But the answer given is
f ( t ) = k ( s ) t ,
where
k ( s ) = s + 4 s 3 s 2 2 .
If anyone can provide me some hint on how to proceed and reach the specified answer, I would be really grateful.

Answer & Explanation

Monserrat Cole

Monserrat Cole

Beginner2022-07-03Added 12 answers

The answer given, f ( t ) = k ( s ) t , where
k ( s ) = s + 4 s 3 s 2 2 ,
is correct, as you can verify by differentiating and plugging into the DE. It is evident by inspection that k ( s ) t satisfies the IC.
As for finding the solution, guess-and-check proves to be the best method. You can see by inspection that some constant (that is, some quantity constant with respect to t) times t might be able to solve the DE, and it certainly satisfies the IC. So, posit f ( t ) = k ( s ) t , plug it into the DE, and solve for k ( s ) ::
s ( 1 s ) t = k ( s ) ( k ( s ) t s t ) s ( 1 s ) = k ( s ) ( k ( s ) s ) for t 0.
Now solve for k ( s ) , (might need the quadratic formula) and you're finished.

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