**Due Friday, September 29**

1. In the proof of Proposition 16 on page 12 of the text, the last sentence reads "A similar argument using a bisection of the side BC results in the conclusion that angle DCA > angle ABC." Write in detail the omitted proof to which this statement is referring. (Note that in the text the inequality sign goes the other direction, but that is clearly a typo.)

2. Write proofs of Propositions 20 and 21. For 20, you have the appendix to refer to. The proof of 21 is not in the appendix; you need to figure it out for yourself! Since you may find the statement of this proposition a little confusing, I will talk about it in class before it is due.

By the way, some of you probably already have a knack for writing
great proofs, while others are a little rusty. For those of you who
had trouble with the first assignment, I strongly suggest you come to my
office to run these proofs by me before handing them in! (You can also
ask some of your classmates to help you critique them before handing them
in.)