I have a rather simple question here which however I feel needs a lot of background info to ask properly. So I apologize in advance!

I'm self-studying with Fitzpatrick'sAdvanced Calculusand am working on a problem from the very first section, where no calculus has been introduced yet:

Define.

a.Show that is an upper bound for and therefore, by the completeness axiom, has a least upper bound that we denote by .

b.Show that if , then we can choose a suitably small positive number such that is also an upper bound for , thus contradicting the choice of as the least upper bound of .

c.Show that if , then we can choose a suitably small positive number such that belongs to , thus contradicting the choice of as an upper bound of .

d.Use parts (b) and (c) and the positivity axioms for to conclude that .

The problem was asked previously in this thread: Upper Bound.

From the hints on that thread I think I have answered parts a-c, however, I have a question on part d. Parts b and c showed that and , respectively. So it seems obvious that must equal , but I am confused by the reference to the positivity axioms. As stated in the books, they are:

1. If and are positive, then and are also positive.

2. For a real number , exactly one of the following three alternatives is true: is positive, is positive, .

How do these allow us to conclude that $b^2=c$? Sorry for the involved question, it's probably really simple but I have no one to ask.