Adding Integers and Subtracting Integers

In this lesson we will look at adding integers and subtracting integers using the number line and using rules.

Using the Number Line

When we add, we move right on the number line.

When we subtract, we move left on the number line

Adding Integers

Example:

To evaluate –3 + 2 , we start at –3 and move 2 places to the right

The answer is –1

Subtracting integers

To evaluate –3 – 2 , we start at –3 and move 2 places to the left

The answer is –5

The following video will show an example of adding integers and subtracting integers using the number line.

Using Rules

Using the number line to add or subtract integers may be quite tedious. A quicker method would be to use the following rules:

Rule 1 :

The sum of two or more positive integers is a positive integer.
The sum of two or more negative integers is a negative integer.

Example:

(+7) + (+ 4) = + 11
(– 8) + (– 9) = – 17

Rule 2 :

To find the sum of a positive and a negative integer:
Subtract the two numbers (ignore the signs) and then keep the sign of the larger integer.

(– 9) + (+ 7) = – 2
(– 3) + (+ 5) = + 2

Rule 3 :

The sum of an integer and its opposite is equal to zero

(– 9) + (+ 9) = 0
(– 3) + (+ 3) = 0

The following video will show some examples of using the rules to add intgers and to subtract integers.

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