Fundamentals of Mathematics, 2008a

593
(2 positive units) + (3 positive units) = (5 positive units)
The addition of the two negative numbers -2 and -3 is performed on the number line as follows.
Begin at 0, the origin.
Since -2 is negative, move 2 units to the left.
Since -3 is negative, move 3 more units to the left.
We are now located at -5.
Thus, (−2) + (−3) = −5.
Summarizing, we have
(2 negative units) + (3 negative units) = (5 negative units)
Observing these two examples, we can suggest these relationships:
(positive number) + (positive number) = (positive number)
(negative number) + (negative number) = (negative number)
Adding Numbers with the Same Sign
Addition of numbers with like sign:
To add two real numbers that have the same sign, add the absolute values of the numbers and associate
with the sum the common sign.
10.5.2.1 Sample Set A
Find the sums.
Example 10.21
3 + 7
|3| = 3
|7| = 7
} Add these absolute values.
3 + 7 = 10
The common sign is +.
Thus, 3 + 7 = +10, or 3 + 7 = 10.
Example 10.22
(−4) + (−9)
| − 4| = 4
| − 9| = 9
} Add these absolute values.
4 + 9 = 13
The common sign is −.
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