I read "No. 4"--Simplest Cases. This is just like how I remember calculus from high school--a bunch of rules that I memorized but had not idea how to apply to real-world scenarios. It's all about x, dx, y, dy, etc.

Maybe it's easy, but it's hard for me to know how its useful.

I think mathematicians who write math instruction love math for maths sake. The rest of us want to know how we can use it to solve problems we face in our everyday lives.