**Due Wednesday, September 19**

1. This problem concerns Euclid's Proposition 16, which asserts the following: Suppose we are given triangle ABC, with BC extended to D. Then: (1) angle DCA is greater than angle BAC and (2) angle DCA is greater than angle ABC. In class, we proved (1). It is your job to prove (2).

2. Write proofs of Propositions 20 and 21.

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.)