Now I can finally integrate for fun without running into a dead end just because I forget something like the bell curve function. I just said integrate for fun. I'm gonna go now.

Update: The one by Keisler seems to be the most interesting. The concept of infinitemsimals is developed before integration/differentiation, which is something I would have like during my days studying calculus. It seems like such a great way to really develop the concept of an infinitesimal by itself rather than have it as a limit/side product of more finite processes like taking the slope of a line. Go Keisler. I think I'll read through this text and get my integration on! Or not. We'll see how bored I get with life...