How Do You Solve the Differential Equation x'=x(M-x) for x(t)?

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Discussion Overview

The discussion centers around solving the differential equation x' = x(M - x) for x(t), which is framed within the context of population growth modeling. Participants explore various methods and approaches to find a solution, including analytical techniques and potential behaviors of the solution.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in solving the equation and seeks assistance.
  • Another participant suggests that there may not be an analytical solution and mentions the possibility of chaotic behavior depending on the parameter M.
  • A different participant believes the equation is separable.
  • One participant proposes a direct integration approach and provides a derived expression that could lead to x(t) by solving a quadratic equation.
  • Another participant attempts to use partial fractions for integration but initially finds it unworkable, later revising their approach and arriving at a solvable form.
  • A participant acknowledges a mistake in their earlier work by noting a sign flip.

Areas of Agreement / Disagreement

Participants express differing views on the solvability of the equation, with some believing it is separable while others question the existence of an analytical solution. The discussion remains unresolved regarding the best approach to take.

Contextual Notes

Some participants' approaches depend on assumptions about the parameters involved, and there are unresolved mathematical steps in the integration processes discussed.

AronH
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I was making a problem about population grow, and I wasn't able to solve this:
x'=x*(M-x), for x(t).
Can anyone help me?
Thanks.
 
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I have the impresion that there is no analytical solution to that one. IIRC, its solution has a chaotic behavior depending on M (though there may be another parameter).
 
I think it's seperable.
 
Integrate directly to find
[tex]\frac{x}{x_0} \times \frac{M-x}{M-x_0} = e^{M t}[/tex]
from which you can find x(t) by solving the quadratic equation.
 
scribbly scribbly...

[tex]\frac{dx}{dt}=xM-x^2[/tex]
[tex]\int \frac{dx}{xM-x^2}=\int dt[/tex]

... doesn't work, never mind...

edit: wait,wait,wait... partial fractions:

[tex]\frac{1}{xM-x^2}=\frac{1}{Mx}+\frac{1}{M(M-x)}[/tex]

[tex]\int \frac{dx}{Mx}+\int \frac{dx}{M(M-x)}=\frac{1}{M}(\ln(\frac{x}{x_0})-\ln(\frac{M-x}{M-x_0}))=\frac{1}{M}\ln\left(\frac{x(M-x_0)}{x_0(M-x)}\right)[/tex]
So:
[tex]\frac{x}{x_0}\frac{M-x_0}{M-x}=e^{Mt}[/tex]
Look solvable now...
 
Last edited:
Thanks - I flipped a sign!
 

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