From: utzoo!decvax!harpo!ihnp4!ixn5c!inuxc!pur-ee!uiucdcs!becker Newsgroups: net.math Title: = .999... neato proof - (nf) Article-I.D.: uiucdcs.1399 Posted: Thu Jan 27 18:35:37 1983 Received: Sun Jan 30 10:43:09 1983 #N:uiucdcs:28200006:000:1128 uiucdcs!becker Jan 27 17:03:00 1983 I found a really neat proof in a book called "What Are Numbers" by Louis Auslander; however, my roommate doesn't believe it, either. notation: b(i) is a subscripted digit. There might be a more posh way to type this, but I don't know how. <> means 'is not equal to'. here goes: consider the infinite decimal a = .9999... we want to find a number b = b(1)b(2)b(3)b(4)... such that a = 1 - b if b(1) <> 0, then a > .99 and b >= .1, so a + b > .99 + .1 (= 1.09). therefore, b(1) = 0. if b(2) <> 0, then a > .999 and b >= .01, so a + b > .999 + .01 (=1.009). therefore, b(2) = 0. moving along in such a fashion, we come to conclude that b = 0, and, since we are doing arithmetic with infinite decimals, 1 = .999... I like this proof alot. My roommate maintains that 1 - .999... = 9 * 10^infinity, which is sorta humorous considering that the first proof I showed him involved multiplying .999... by 10. The only advice I can give is : Never Room With An Architecture Major. (no flames please) cheers, craig