91 - 92 = ?Date: 01/23/97 at 22:54:40 From: Garry Griffin Subject: 91-92 91 - 92 = ? Date: 01/26/97 at 16:23:18 From: Doctor Reno Subject: Re: 91-92 Hi Garry! I assume that you are asking how to subtract a number from a number that is smaller than itself. My students don't think they understand this either! Please notice that I wrote *think*! I live in New England, and last weekend it was extremely cold here. In fact, the temperature dropped to below zero! One night, the temperature was -5 degrees! Pretty nasty out! I mention this to demonstrate to you that you do, indeed, understand how to do the subtraction in your problem...and I will prove it to you! Think of a thermometer....and of an unusual, *very* unusual, winter day. On this strange day, the temperature outside rises to a very hot 91 degrees. You and your friends put on your shorts and turn on the air conditioning! You look at the thermometer and notice that the red mercury inside it has risen up to the mark that says 91. But during this day, clouds roll in and it starts cooling off *very* quickly. This is historic weather that you will tell your grandchildren about, so you spend lots of your time on this day watching the mercury drop in your thermometer. After the temperature has dropped 10 degrees, the thermometer reads 81; after a 25 degree drop, the thermometer reads 66; and when the thermometer has dropped 55 degrees, what will your thermometer read? I hope you answered 36 degrees! What will the the thermometer read when the temperature drops 91 degrees? If you are having problems answering this, draw a picture of a thermometer. Make the little marks for the temperature and you will see that the temperature will be 0 degrees if the temperature drops 91 degrees. Well, Garry, there you are watching the thermometer drop as it gets colder. It is now zero degrees, and you can't imagine it getting any colder, but suddenly, it does! The temperature drops one more degree, so that it is now 92 degrees colder outside than it was when you put your shorts on! What will the thermometer read now? Look at a real thermometer if you are still having problems....you will see that it reads -1 degree. And we *know* how cold that is! My point with this long explanation is to show you that you *do* know how to do subtraction with negative numbers. If you draw a number line, you will see that it is simpler than you thought: <--(-4)----(-3)----(-2)----(-1)----0----1----2----3----4----5--> If we were to do the subtraction problem 5-7 , we would put our pencil on 5 and go back 7 units. What is the answer? Yes, it is -2. To prove that our answer is correct, we could add -2 to 7 by putting our pencil on -2 and counting up 7 units. We return to 5! If you start getting lost, wondering whether you should go left or right on the number line, just remember that addition INCREASES the values of numbers so that you must go right. Subtraction DECREASES the values of numbers, so we go left. (Please note that word processors make drawing number lines difficult, but these numbers are an equal distance apart!) I hope this not only helps you answer this math problem, but also any future subtraction problems that involve negative numbers! -Doctor Reno, The Math Forum Check out our web site! http://mathforum.org/dr.math/ |
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