The following problem has been taken from Paul's Online Notes:
"We need to enclose a rectangular field with a fence. We have 500 feet of fencing material and a building is on one side of the field and so won’t need any fencing. Determine the dimensions of the field that will enclose the largest area."
This problem is easy to model in the real-world because the area of the field will depend upon the length x and the width y.
Therefore, the area of the field can be modeled as function of either x or y: $A(x) = xy$
Likewise, the perimeter can be modeled as a function with the given constraint: $500 = 2x + y$
You can then find the answer by relating the two functions to each other and taking the derivative to find the critical points. This type of problem makes sense to me because the functions that are required are easy to determine.
However, take another problem from Paul's Online Notes, particularly from the section of business applications:
An apartment complex has 250 apartments to rent. If they rent x apartments, then their monthly profit in USD is given by the function:
$P(x)=-8x^2 +3200x-80000$
How many apartments should they rent to maximize their profits?
I do understand the mathematical theory of finding the extrema via critical points, and I also understand the business reasons as to why the maximum profit cannot be equal to the maximum apartments available to rent (i.e. the answer isn't going to be "all of them"). But if I were the owner of this apartment complex, how would I even determine the original function $P(x)$ in order to actually determine my maximum profit?
For other applications such as population modeling, how is the function determined for population growth? I see problems in textbooks such as, "The population of flies grows at a rate of $e^{2t}-19.23$. In how many years will the population..."
How are these functions determined in real-life? And how would they change if something were to happen to the business? For example, how would the function in the apartment complex problems change if they suddenly had 300 apartments available to rent?