Lesson 1  Undefined Terms 
Why do we need to learn proofs?A proof is an argument, a justiﬁcation, a reason that something is true. It’s
got to be a particular kind of reasoning – logical – to be called a proof. In fact, you use “proofs” all the time. When you reason with yourself that it will be cheaper to buy the larger cans of beans, you are proving something about the respective prices. 
