Åk 6–9
 
English/Soomaali
3.2 Calculating with Negative Numbers
When you add and subtract, you can make this more clear by putting all the numbers in parenthesis and by being thorough by writing the sign for both the positive and negative numbers.  In this material, we will also use parenthesis in order to make this extra clear.


Exemple 1:

You had 5 kr and win 3 kr, how much do you then have?

5 + 3 = (+5) and (+3) = (+5) + (+3) = ?

When we show this on the number line, it becomes this:



We go from +5 to +3 steps. We then end up on +8. The answer is then +8 or as we normally write it: 8.

5 + 3 = (+5) and (+3) = (+5) + (+3) = 8

Answer: You have in total 8 kr.


Example 2:

If we instead have 5 kr and lose 3 kr, then it is written like this:

5 - 3 = (+5) and (-3) = (+5) + (-3)

If we show this on the number line, it becomes this:



We go from +5 and take -3 steps. We then end up on +2. The answer is then +2 or as we normally write it: 2.

5 - 3 = (+5) and (-3) = (+5) + (-3) = 2

Answer: You have 2 kr left.


Example 3:

The temperature in the morning is -3ºC. During the middle of the day, the temperature rises 4ºC. It has become 4ºC warmer. What does the thermometer then show?

-3 + 4 = (-3) and (+4) = (-3) + (+4) = ?

On this number line, it looks like this:

We go from -3 and take +4 steps. We then end up on +1. The answer is +1 or as we normally write it: 1.

-3 + 4 = (-3) and (+4) = (-3) + (+4) = 1

Answer: In the middle of the day, the temperature is +1ºC or 1ºC.



Example 4:

You owe 4 kr and borrow an additional 2 kr. How much do you now owe? (Here, we express debt with a negative number.)

-4 - 2 = (-4) and (-2) = (-4) + (-2) = ?

We look at the number line:



We go from -4 and take -2 steps. We then end up on -6. The answer is then -6.

-4 - 2 = (-4) and (-2) = (-4) + (-2) = -6

Answer: You owe 6 kr.