Review of differential equations

This is a very brief review of differential equations. I hope you have some textbooks to fill in all the details.

Suppose we have a variable x -- it could represent time, or distance, or any other property. We discover that there's a connection between this variable and another quantity: the other quantity depends on x. We can call it a function of x: f(x).

There are many ways that we can connect two quantities mathematically. Let's start with simple ones and work our way up to more complex ones.