This course has a staggered video lecture / exercise format. The first module had me use datapoints to come up with a differential equation, it took lots of mistakes and backtracking, but I got about 60% of the answers right. Module 2 went over a lot of ground rules.
A first-order differential equation looks something like: dy/dt = 3y
they call it first-order because its based on a first-derivative
If you can separate the variables: (1/3y)dy = dt
then you can integrate both sides, use some algebra, and nearly solve for one in terms of the other: y = (e^3c * e^3t)/3
An initial-value (a known y which corresponds to a known t) can clean up the equation a little and allow you to solve for the unknown constant (c in this case).
Whatever the answer is, you should be able to check it against the initial equation (dy/dt = 3y) by:
1. Plugging in your solved value of y into dy/dt = 3y
2. Taking the derivative of your solved value
If these aren't the same, then you don't have a general solution and you probably did something wrong
I'm taking this because I don't want my math progress to stagnate. This is being taught for free on edx.
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I'm a math evangelist, I believe in theorems and ignore the proofs.