Tldr version below.
Several (6+) years ago I was failry interested in 2d game design and I had a couple 2d tile based game engines fairly complete for my needs. At some point I became more interested in asset creation (2d/3d art) and stopped coding. I recently went back to coding and quickly realized that the majority of the geometry and algebra maths that I had used frequently then had found their way out of the confines of my thick skull.
Can anyone suggest a few good sites with practical examples of some of the more common equations used in 2d game development?
My biggest hurdle thus far is working out collision detection. Circle vs circle and rect vs rect are easy.. More complex/mixed geometry is a different story and I know the further I get into coding the more lost knowledge I'm bound to encounter.
They always say 'use it or lose it' I lost it. >.<
Well, I suppose that the main things you need are vector and matrices (linear algebra) and angles stuff (trigonometry)... I don't know of any site that collects together 2d math ready for use, but math sites as http://mathworld.wolfram.com/ or even wikipedia could be good starting points...
You couls also use google 'code search' or similar tools to look for libraries and algorithm sources.
Programming isn't about math, it's about your head. People that are used to do math and started to do more math than coding had problems when they were supposed to return to coding and be productive. They lost skills and ability to churn sufficient quantity of code with ease.
As for your problem, self learn everything, or learn 3D math, and kill an axle.
Try the book "Mathematics and Physics for Programmers" by Danny Kodicek. It discusses collision detection and response quite reasonably in a language-agnostic way (lots of pseudocode). Also includes some discussions about tile engines, physical collision response etc. It's not the most detailed, modern or specific treatment, but might prove useful based on your post.