Well, I'm back. 🙂 I'm attaching another example. Example D is the same as the previous sample I sent, so it will be the same formatting. However, Example E is just the opposite. At first glance, it looks like special margins are required, but the last line prohibits that. In that case, I will use 3-5 margins for the displayed math here too. Now I'm concerned about consistency so am wondering if I can and should include the general omission symbol to the left of the equal signs for the 4 middle lines of math. It feels strange to do that, but these are all examples of the same type of calculations.
I included the paragraph that explains what is being demonstrated. I think they don't repeat the missing side because they are mimicking what one would do if they were calculating this on paper.