Module 2 · Signed numbers · Lesson 4 of 39
Adding and subtracting signed numbers
Rules for every combination of positive and negative numbers.
In this lesson · about 16 minutes
- Add two numbers with the same sign or different signs
- Rewrite subtraction as adding the opposite
- Know when order matters
Signed numbers are positive and negative numbers. When we add two of them, there are three cases.
| Case | The trick | Example |
|---|---|---|
| Positive + positive | Add, keep the sign positive | 3 + 4 = 7 |
| Negative + negative | Add as if both were positive, then make it negative | −3 + (−4) = −7 |
| One of each | Subtract the smaller absolute value from the larger. The answer takes the sign of the number with the larger absolute value | 3 + (−4) = −1 10 + (−1) = 9 |
If the two numbers are opposites, the sum is 0: 3 + (−3) = 0.
Worked example
Follow −10 − (−1)
Simplify −10 − (−1).
Try it · interactive
Number line: add and subtract signed numbers
Check each rule by moving on the number line. Adding moves right, subtracting moves left, and subtracting a negative moves right.
−3 + 5 = 2
Move 5 to the right.
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