# Show directly that P is a solution to the differential equation

1. Jun 3, 2012

### phosgene

1. The problem statement, all variables and given/known data

Show directly that
$P=\frac{L}{1+Ae^{-kt}}$

is a solution to the differential equation

dP/dt=kP(1-P/L)

2. Relevant equations

-

3. The attempt at a solution

I assume that all I need to do is differentiate P with respect to t. However, as you can see below, either I'm doing it wrong or I don't know what I need to do next.

$dP/dt=(d/dt)\frac{L}{1+Ae^{-kt}}$

First I take 1+Ae^-kt to be 1/x, and differentiate it to -x^-2. Then I multiply it by the derivative of x, which is -Ake^-kt. So I finish up with $\frac{LAe^{-kt}}{(1+Ae^{-kt})^{2}}$

Im stuck here. I have no idea how to get the correct answer :S

2. Jun 3, 2012

### SammyS

Staff Emeritus
You're missing k in that last expression:
$\displaystyle \frac{kLAe^{-kt}}{(1+Ae^{-kt})^{2}}$​

Multiply the numerator & denominator by L .

Change the $Ae^{-kt}$ in the numerator to $\left(1+Ae^{-kt}\right)-1$

3. Jun 3, 2012

### phosgene

Thanks! I'll try it out and see if I can get it.