I have a very short question on this proposition : http://aleph0.clarku.edu/~djoyce/java/elements/bookI/propI17.html
I understand the way the theorem was proved. Euclid proves that angles ABC and ACB is less than 180 degrees. At the end, he says the same method can be applied to prove for the other cases. I was able to do the same for the angle BAC and ACB. But how do I do it for CAB and ABC ? It doesn't seem the same method holds for this case, am I right ?