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 +  6

ii. 2 + 7

iii. -6  –  4

iv. -8  +  11

v. -5   +  4

vi. -3  –  5

Solution

i. 0   +   6

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

So,   0  +  6   =  6

ii. 2   +   7

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

So,  2  +  7  =  9.

iii. -6  –  4

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

So, -6  -4     =   -10.

iv. -8   +  11

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

v. -5    +     4

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

vi.  -3  –  5

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

So,   -3   – 5   =   -8

Note:

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

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