Tips for Negative and Positive NumbersDate: 11/30/2001 at 17:47:46 From: Chris Subject: Tips for negative and positive numbers Integers are tricky I get multiplying and division but subtracting and adding them confuses me. I really get confused when adding or subtracting a negative and a positive. Can you give me tips on how to add and subtract integers? Thank you. Date: 11/30/2001 at 19:08:59 From: Doctor Achilles Subject: Re: Tips for negative and positive numbers Hi Chris, Thanks for writing to Dr. Math. When it comes to multiplication and division, it can get a little tricky. But you can use a pretty simple rule: count up the number of negatives. If you have an EVEN number of negatives (0, 2, 4, etc.) then the answer will be POSITIVE; if you have an ODD number of negatives (1, 3, 5, etc.) then the answer will be NEGATIVE. What I do with addition and subtraction is a little different from what most people do. Let me take a roundabout way to get there and show you. I hope this will be helpful. Addition is easier than subtraction for me. The reason is that addition is symmetric or commutative. (See the Dr. Math Glossary of Properties and select "commutative": http://mathforum.org/dr.math/faq/faq.property.glossary.html .) For example: 1 + 2 is the same thing as: 2 + 1 Subtraction is NOT symmetric: 1 - 2 is NOT the same as: 2 - 1 When you start dealing with negative and positive integers, and adding and subtracting, things start getting really hard. I don't like memorizing a lot of different rules; I only want to memorize one or two rules if I can. So these are the rules I know: 1) I know how to translate any addition or subtraction of integers problem into an addition problem. 2) I know how to add positive and negative integers. This way I don't have to learn how to subtract positive and negative integers and get confused with that. The way to translate a subtraction problem to addition is this: Whenever you see a minus, change the sign integer that comes right after it and make the minus into a plus. So when you have a minus followed by a negative number, for example: 3 - (-5) = ? just CHANGE the -5 to 5 and change the minus to a plus: 3 + 5 = ? And when you have a minus followed by a positive number, for example: 4 - 6 = ? just CHANGE the 6 to -6 and change the minus to a plus: 4 + (-6) = ? The reason I like this form is that then I don't have to worry about which number comes before and after the + sign. All I have to know is that I have a 4 and a -6 and I'm adding them together. It doesn't matter to me whether the actual problem I have is: 4 + (-6) = ? or: (-6) + 4 = ? I just need to know which two numbers I'm adding together and that's all. Okay, so much for rule 1 (changing to addition). Now how do I add integers? (Rule 2). All I do here is just count. I count (add up) how many positives I have and count (add up) how many negatives I have. Then I imagine that they are on two teams, fighting each other. Each time a negative finds a positive, they cancel each other out, and then I just see what's left over. So with: 2 + 3 = ? I have 5 positives and no negatives, so when they get together there are no negatives so the 5 positives all make it and the answer is 5. With: (-1) + (-3) = ? I have no positives and 4 negatives, so when they get together there are no positives so the 4 negatives all make it and the answer is (-4). With: 4 + (-6) = ? I have 4 positives and 6 negatives. When they get together, one negative cancels out one positive: that leaves me with 3 positives and 5 negatives. Then another negative cancels out another positive: that leaves me with 2 positives and 4 negatives. Then another negative cancels out another positive: that leaves me with 1 positive and 3 negatives. Then another negative cancels out another positive: that leaves me with 0 positives and 2 negatives. Now we're out of positives, so the negatives win and there are 2 of them left, so the answer is (-2). Finally, with: (-3) + 8 = ? I have 8 positives and 3 negatives. When they get together, one negative cancels out one positive: that leaves me with 7 positives and 2 negatives. Then another negative cancels out another positive: that leaves me with 6 positives and 1 negative. Then another negative cancels out another positive: that leaves me with 5 positives and 0 negatives. Now we're out of negatives, so the positives win and there are 5 of them left, so the answer is 5. I hope these rules and examples were helpful. If you're still stuck, please write back and we'll try to help you some more. - Doctor Achilles, The Math Forum http://mathforum.org/dr.math/ |
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