Linear Algebra, I am in You
Speaking of coordinate systems, the following 3 part primer from the Wolfire Dev Team is an excellent refresher on linear algebra. I studied this in school, but having been out of the engineering game for a while I find my “bow staff skillz” are shamefully rusty.
Fortunately, Cocos2d takes care of a lot of this for you with their CGPoint Extensions. But, it’s still good to understand the basics.