Math 6High (Period 3)

Subtracting Integers 4.4

So to generalize the rules we discussed in class, when using a number line to show subtraction (Looking at Page 181 at the number line examples):

To subtract a positive integer , move in the negative direction

To subtract a negative integer, move in the positive direction.

ADD The Opposite!
We had a huge discussion about way too pessimistic people and how we wish we could take away a little of their negativity… and  about too positive people—even someone in our class [ :) ] whom some of us  wished was just a little less positive!! We talked about how instead of taking away a little – if we gave a little of the opposite—that might work as well… and we realized that this applies to math as well.

ADD THE OPPOSITE

That is, instead of subtracting a number—add it’s opposite. 

Then use the rules for adding integers that you have learned from the previous lessons

In general:    a - b = a + (-b)

Several examples:

6 – 8 = 6 + (-8)  In this case by adding the opposite, you are adding two integers with different signs—so you following the adding integers with DIFFERENT SIGNS RULE. Ask “Who wins?” and “by how much?”  The negative wins by 2 so the answer is -2              

6-8 = -2

-9 - 10 =  -9 + (-10) In this case you are adding two integers with the same sign. So use the SAME SIGN RULE.  -9 + (-10) = -19

What about

54- (-12) = 54 + (+12) = 54 + 12  In this case you are adding two integers with the same sign. So use the SAME SIGN RULE.  54 –(-12) = 66

We talked about  making sure to double check—that is, check, check—both  places- where you change the subtraction sign to a positive AND where you change the second numbers sign to its opposite.