Is it really on topic to ask questions regarding an exact proof/how to approach the proof of a particular identity, or a physical equation?
Simply asking "How does line X.Z come from line X.Y?" is likely to get your question closed as homework-like (or maybe unclear if you don't quote the relevant section of the text/paper you're referring to). So for it to remain open and answered, it needs to be framed to be about the physics of what the math is saying, rather than how to do the math.1
For example, one could ask about the assumptions underlying the derivation, but I think it important to include your expectation of why you think something is screwy with the derivation (e.g., Why do we take the small x limit here, can't x be big because Reason Y?).
1. Such a question would likely be on topic over at Math.SE.