# Thread: Word Problem - Integration

1. ## Word Problem - Integration

Hi,

I don't think I know how to do all of this yet however:

The evolution of a population of rhinoceros, R(t), in hundreds, time in years, in anAfrican National Park is given by the equation,

dR/dt =1/5R(2 − R); R(0) = 0.5

(a) Solve the system exactly for R(t).
(b) What happens as the time t → ∞, i.e. what is the population a long time in the future?
(c) Write an Euler scheme and compute until the population levels off (using Excel, Matlab,Octave, LibreCalc or similar).
(d) Plot both of your solutions on the same set of axes and comment.

(a) dR/dt = 1/5 R(2-R)
Multiply R through brackets - dR/dt = 1/5 2R + R2 or dR/dt = 2R+R2/5
Divide both sides by dR - dt = (2R+R2)/5

If I am correct I can see dt = a number plus its derivative - does this mean I can use integration by substitution setting R2 as the number to be substituted? Or am I on the complete wrong planet?

Also for(b) am I looking for the absolute maxima of the equation?

Kind regards
Beetle

2. ## Re: Word Problem - Integration

For part (a) - I meant to say - is that just the product rule backwards?

3. ## Re: Word Problem - Integration

Just want to check the function. Is it $\displaystyle \frac {dR}{dt} =\frac{1}{5}R(2-R)$ or is it $\displaystyle \frac {dR}{dt} =\frac{1}{5}t(2-t)$?

4. ## Re: Word Problem - Integration

Hi, it is the first one.
Cheers
Beetle

5. ## Re: Word Problem - Integration

Originally Posted by Beetle37
For part (a) - I meant to say - is that just the product rule backwards?
Integration by separation of variables, then partial fraction decomposition ...

6. ## Re: Word Problem - Integration

Again (I hate to say it), have a look at the example in:
https://en.wikipedia.org/wiki/Separation_of_variables

For (b) you need to consider the limit of the function R(t) as t ->infinity.

7. ## Re: Word Problem - Integration

dR/dt =1/5R(2 − R); R(0) = 0.5
$\dfrac{dR}{R(2-R)} = \dfrac{1}{5} \, dt$

partial fraction decomposition ...

$\dfrac{1}{R(2-R)} = \dfrac{A}{R} + \dfrac{B}{2-R}$

$1 = A(2-R) + BR$

$R = 0 \implies A = \dfrac{1}{2}$ ; $R=2 \implies B = \dfrac{1}{2}$

$\displaystyle \dfrac{1}{2} \int \dfrac{1}{R} + \dfrac{1}{2-R} \, dR = \dfrac{1}{5} \int \, dt$

$\displaystyle \int \dfrac{1}{R} - \dfrac{-1}{2-R} \, dR = \dfrac{2}{5} \int \, dt$

$\ln|R| - \ln|2-R| = \dfrac{2}{5} t + C$

$\ln\left|\dfrac{R}{2-R}\right| = \dfrac{2}{5} t + C$

$\dfrac{R}{2-R} = e^{2t/5} \cdot e^C$ ; let $e^C = A$

$\dfrac{R}{2-R} = Ae^{2t/5}$

$R = 2Ae^{2t/5} - RAe^{2t/5}$

$R + RAe^{2t/5} = 2Ae^{2t/5}$

$R(1 + Ae^{2t/5}) = 2Ae^{2t/5}$

$R = \dfrac{2Ae^{2t/5}}{1 + Ae^{2t/5}}$

$R(0) = \dfrac{1}{2} \implies A = \dfrac{1}{3}$

$R = \dfrac{2e^{2t/5}}{3 + e^{2t/5}}$

$R = \dfrac{2}{1 + 3e^{-2t/5}}$ ... a logistic curve

$\displaystyle \lim_{t \to \infty} \dfrac{2}{1 + 3e^{-2t/5}} = 2$

8. ## Re: Word Problem - Integration

shortcut ...

general model for logistic growth is

$\dfrac{dy}{dt} = k\left(1 - \dfrac{y}{M}\right) \implies y = \dfrac{M}{1 + Be^{-kt}}$ where $M$ is the carrying capacity for $y$ and $B = \dfrac{M-y(0)}{y(0)}$

transforming $\dfrac{dR}{dt} = \dfrac{1}{5}R(2-R)$ to the form shown above ...

$\dfrac{dR}{dt} = \dfrac{2}{5}R\left(1-\dfrac{R}{2}\right)$

$k = \dfrac{2}{5}$, $M = 2$, $B = \dfrac{2-0.5}{0.5} = 3$

$y = \dfrac{M}{1 + Be^{-kt}} \implies R = \dfrac{2}{1+3e^{-2t/5}}$