Lecture 13 We continued discussing Book I of Euclid's Elements with our ultimate goal of proving the Pythagorean Theorem. In particular we went over the proofs of the following Propositions: Proposition 4 (SAS = side angle side) Proposition 5 (Thales' Theorem - or - "pons asinorum") Proposition 13 Proposition 15 Proposition 16 Click on the above links to go directly to the proofs and commentaries of the results.