Subtracting Negative Integers - Number LineDate: 10/15/96 at 19:57:18 From: Anonymous Subject: integers How do you subtract integers like 5-(-6) ? Anne-Marie New Paltz MS New Paltz NY Date: 12/12/96 at 22:11:30 From: Doctor Sydney Subject: Re: integers Dear Anne-Marie, Let's use the number line for this. -+---+---+---+---+---+---+---+---+---+---+---+---+---+---+--- -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 When we add two positive numbers, we can think of this on the number line as corresponding to starting at one of the numbers and moving to the right from that number a distance equal to the other number. Think about the sum 5 + 2. From the number 5 we have to hop two places to the right. We hop two places to the right because we are adding two more places. For the answer to 5 + 2, we will land on the number 7. -+---+---+---+---+---+---+---+---+---x->-+->-x---+---+---+--- + -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 So 5 + 2 = 7. Now let's think about subtraction: 5 - 2 = ? From the number 5 we need to hop two places to the left. (This time we have to take away from a number, so we hop to the left.) We will land on the number 3. -+---+---+---+---+---+---+---x-<-+-<-x---+---+---+---+---+--- + -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 So 5 - 2 = 3. So, in summary, when we add two numbers, we start at one of the numbers and hop to the right the value of the other number. When we subtract numbers, we start at the first number and move to the left the value of the second number. But what if we are subtracting a NEGATIVE number? This is the question you were asking. For instance, consider the following problem: 5 - (-2) = ? Well, we know from the work we did above that when we subtract a number from another number, we hop to the left the value of the second number. So, to figure out what 5 - (-2) is, we would "hop to the left -2 hops." But what does that mean? We know that hopping to the left 2 hops corresponds to moving to the left 2 hops, but the negative sign reverses everything, so moving to the left -2 hops really means that we move to the right +2 hops! So, 5 - (-2) = 5 + 2. Therefore, from number 5, we have to hop two places forward. We will land on the number 7. -+---+---+---+---+---+---+---+---+---x->-+->-x---+---+---+--- -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 For your sum of 5 -( -6) I am sure you have the idea by now. -+---+---+---+---+---+---+---+---+---x->-+->-+->-+->-+>--+->-x---+--- -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 5 -(-6) = 5 + 6 = 11 Once you understand the basics and practice them often enough, these types of problems will be easy. -Doctors Lim and Sydney, The Math Forum Check out our web site! http://mathforum.org/dr.math/ |
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