Differential Equations: Population Growth: Logistic Equation: Example 1 (Notes)

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Differential Equations: Population Growth: Logistic Equation: Example 1 (Notes)
https://youtu.be/FMFTLa8URDg

In this video I go over an example on the Logistic Equation for Population Growth and this time analyze a direction field for the equation. The direction field is a good way of seeing how many different solutions to the differential equation behave. The interesting characteristics of the directional field is that the solutions move from the P(t) = 0 equilibrium solution to the P(t) = K (the carrying capacity) equilibrium solution. Also the highest growth rate occurs at half the carrying capacity. The last characteristic I will prove in my next video so stay tuned for that!

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# Example: 

Draw a direction field for the logistic equation with k = 0.08 and carrying capacity K = 1000.

What can you deduce about the solutions?

![](https://cdn.steemitimages.com/DQmNfNxJSNbZvNymoUhmScnt9nGAeyu4Ck8D7t2XhmL6xc3/image.png)

The logistic equation is autonomous (dP/dt depends only on P, not t), so the slopes are the same along the horizontal line.

As expected the slopes are positive for 0 < P < 1000 and negative for P > 1000.

The slopes are small when P is close to 0 or 1000 (the carrying capacity).

Notice that the solutions move away from the equilibrium solution P = 0 and move toward the equilibrium solution P = 1000.

![](https://cdn.steemitimages.com/DQmcVRjZBrHJYAZWrLMZu5sqwqaUtm1VtNVigmaiMdyGJrs/image.png)

Let's use the direction field to sketch solution curves with initial populations P(0) = 100, 400, and 1300.

Notice that solution curve that start below P = 1000 are increasing and those that start above P = 1000 are decreasing.

The slopes are greatest when P ≈ 500.
- In fact we can prove that all solution curves that start below P = 500 have an inflection point when P is exactly 500.
- Recall that an inflection point is when the 2nd derivative changes signs, i.e. changes concavity.
- I will go over this in my next video, so stay tuned!

![](https://cdn.steemitimages.com/DQmSj5X8P5XkSaJj18znb9ySeYCvbGVGvtL65Yh5VQdFKjJ/image.png)
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