Introduction to Negative Numbers
Date: 08/11/98 at 17:27:19 From: Jacob IeMaster Subject: Subtraction Dear Dr. Math, I want to know what does 11 - 12 equal? Sincerely, Jacob
Date: 08/12/98 at 13:36:05 From: Doctor Peterson Subject: Re: Subtraction Hi, Jacob. That's a good question. It's easy to say what 12-11 is, so shouldn't 11-12 have an answer too? Until recently, my own 6-year-old daughter would have written 11-12 = C, which stood for "Can't do it." For some questions, that's the best answer you can give. Suppose you had 11 carrots on your dinner plate, and because I love carrots so much, while your head is turned I try to take 12 carrots from your plate. I can't do it! But suppose you had 11 dollars in your bank, and you wanted to buy a game that costs 12 dollars. You can't do it, but I might be kind enough to lend you a dollar. Then you would spend all 11 of your dollars, and you would owe me another dollar. You would actually have one dollar less than nothing, because as soon as you earned another dollar, it would go to me and you would have nothing! Several hundred years ago mathematicians realized that there were a lot of problems they could solve if they had a way to talk about numbers less than zero. So they decided to write 0 - 1 = -1, which is read as "negative one" and means "one less than zero." From what I just said about owing a dollar, you can see that: 11 - 12 = -1 (when you spend 12 dollars, you owe me one) and -1 + 1 = 0 (when you earn another dollar, you will have nothing) There's actually another place where you can see negative numbers very easily. Look on a thermometer, and you'll see that the temperature can go down to zero degrees, but then it can keep getting lower. Temperatures below zero are written as negative numbers, like -10 degrees which means 10 degrees below zero. If the temperature now is 11 degrees, and it gets 12 degrees colder, it will be -1 degrees. - Doctor Peterson, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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