Subtracting Negative IntegersDate: 09/12/2001 at 22:27:42 From: Dawn Contois Subject: Subtract Integers I'm having some trouble with subtracting integers. My teacher explained, but I just can't figure it out on my own. I need some help. I have questions like: -3 - -13 My teacher said to do it in a money way, like you owe $13 and you also owe $3 so add them and you get $16 in debt = -16. Is that right? Thanks a lot. Date: 09/12/2001 at 23:02:52 From: Doctor Peterson Subject: Re: Subtract Integers Hi, Dawn. That would be right if you were ADDING -3 and -13; but you are supposed to SUBTRACT. Suppose today you owe $3, but yesterday you owed $13. The difference is how much money you gained between yesterday and today. Since you owe $10 less now than today, you must have gained $10; -3 - -13 = 10. You can also do this on the number line: -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 <--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+--> +-------------------------------------->| 10 To get from -13 to -3, you go 10 units to the right, so -3 - -13 = 10. Yet another way to do this is to recognize that subtraction is the same as addition of the negative, so -3 - -13 = -3 + -(-13) = -3 + 13 = 13 + -3 = 13 - 3 = 10 Do you see the steps there? I replaced subtraction with addition, then replaced a double negative with a positive, then reversed the order and subtracted. This is how I think when I do this; but it takes time to get used to this sort of manipulation. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ |
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