Book Five, Chapter 3

In class we discussed part of the second paragraph of Chapter 3 in Book 5. However, to be quite honest, the beginning of the chapter and from 1131a, line 30, to about the bottom of the page I just found myself becoming confused. I think it's mostly the language, not necessarily the context. I'm reading a great deal of repetition in the beginning. Then, from line 30 to the bottom of the page, throwing in the mathematical references just confuses me. So, in short, if someone could just clarify that section in a nutshell. I understood the chapter overall fairly well, I felt; but I just found myself scratching my head at that section. Thanks!