Download Introduction to Signed Numbers

Subtraction with Signed Numbers
The easiest way to subtract signed numbers is to change the subtraction problem to
addition. We can do this because subtracting a number is the same as adding its opposite.
For example:
65 1
6   5  1
9  12  3
9   12   3
Example
 6   4
In other words, to subtract signed numbers:
Step 1 – Copy the first number down as is.
6
Step 2 - Change the subtraction sign to an addition sign
6
Step 3 – Change the sign of the second number to its opposite
6
Step 4 – Add the new numbers by using your rules for adding
43
More examples:
 4   3
7
 11   11
12  3
 11  11
15
0
 5  9  ____
3  13  ___
 4   3  _____
5   11  ____
 2   7  ____
 14  14  ____
Hot Air Balloon Model:
A model that might help you visualize subtraction with
signed numbers is a hot air balloon. Think of positive
numbers as being hot air-bags and negative numbers as being
sand-bags. First, think of addition. When you add air-bags
(positives) to your hot air balloon it goes up. When you add
sand-bags (negatives) to your balloon it goes down. Now think
of subtraction. If you take away (subtract) hot air-bags
(positives) from your balloon, it would cause the balloon to go
down. If you take away (subtract) sand-bags (negatives) it
would cause the balloon to go up.

2
12   3
Now you try:
 4
5
4
3
2
Subtracting 5 sand-bags
and adding 5 air-bags have
the same affect
1
 1   5
0
1 5
-1
4
-2
-3
-4
-5