I hope to be using this site appropriately; otherwise, just disregard. I have used statistics in research for a long time, but don't have a background in advanced math or calculus. Curiosity has led me to look into the underpinnings of the bell curve, and there is a step I don't get. I'm not necessarily asking for a detailed answer, but rather some pointers of what I need to study to understand what's going on in the following equations (clearly, what follows is just a fragment of the derivation on a public online document). Thank you!