Lesson 10, Topic 2
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# Addition and Subtraction of Directed Numbers

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In a horizontal number line:

1. When adding, start from the first number, count right along the number line.

2. When subtracting, start from the first number, count left alone the number line.

Note:

In a horizontal number line, numbers to the right of zero are positive, and numbers to the left are negative.

Example 4

Use number line to find the value of:

i. 0 +Â  6

ii. 2 +Â 7

iii. -6Â  –Â  4

iv. -8Â  +Â  11

v. -5 Â  +Â  4

vi. -3Â  –Â  5

Solution

i. 0 Â  + Â  6

Â *Start at 0, count 6 places to the right to end up at 6.

So,   0  +  6   =  6

ii. 2 Â  + Â  7 Â

Â *Start at 2, count 7 places to the right to end up at 9.

So,  2  +  7  =  9.

Â iii. -6Â  –Â  4

*Start at -6, count 4 places to the left to end up at -10.

So, -6  -4     =   -10.

iv. -8 Â  +Â  11

*Start at -8, count 11 places to the right to end up at 3.

v. -5Â  Â  + Â  Â  4

*Start at -5, count 4 places to the right to end up at -1.

vi. Â -3Â  –Â  5

*Start at -3, count -5 places to the left to end up at -8.

So,   -3   – 5   =   -8

Note:Â

The -ve sign directs the number to the left while the +ve directs the number to the right.

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