We’ve talked quite a bit in weeks 3 and 4 here (in particular in the Lectures and Engineering Applied Exercises) about how sometimes a mathematical model is not easily solved analytically or with software. You might wonder: where do these wildly complicated differential equations come from if they’re too complicated  to solve? Well–just because an equation is difficult to solve, that doesn’t mean it was difficult to create. Often we are able to create a differential equation as the starting point, or the most intuitive mathematical description of what we observe in real world phenomenon. A good very simple example of this is the exponential growth model. We might have some familiarity with and its appearance in things like the continuously compounded interest formula or in a basic population growth model–but familiarity isn’t the same as intuition or understanding. When we look at the differential equation, things actually make much MORE sense:
If P is population, t is time, and k is a local, observable growth rate:
Check out the equation:  
This equation tell us that the population grows at a rate that is proportional to the size of the population.
That’s a pretty simple and intuitive thing to write down! However, it’s not until this differential equation is solved that it’s easily useable:
For an explanation of that solution, check out this video! https://www.youtube.com/watch?v=qPzTJeCEAiU