CH 57: This is Q.E.D.
This acronym, which stands for the Latin "quod erat demonstrandum" and means "which was to be demonstrated", is a common way of ending a math proof. It really signaled when proving things in mathematics became less about assertion and more about deduction. This became common during the time of the early mathematicians like Euclid. So when you see Q.E.D. you know you have reached the end of the proof so the author better had proven his point.
Speaking of, I hope that I have demonstrated by this point that math is indeed “fun” or I suppose I will have to get back to work, it is hard out here for a mathematician.