I've spent a good part of the day looking for a post by one of the members which appeared 3 or 4 years ago and which was linked to every so often by other members. The list had about 10 items in it, one of which was the triangle inequality theorem. IIRC the poster said that he/ she referred to this list whenever having a hard time with a proof. If someone will post a link, I promise to print out the post! Thanx!