Lesson 6 of 16
What you may turn round
Show on a big screen8 + 5 is 13. 5 + 8 is 13 too. Are 8 − 3 and 3 − 8 the same?
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Step 1 of 5
- The line: 8 − 3.
- Work it out: 8 − 3 = 5.
- Turn the numbers round: 3 − 8.
- Work it out: 3 − 8 = −5.
- Same numbers, two answers: 5 and −5.
The rule
No. 8 − 3 is 5, but 3 − 8 is −5.
You may add in any order: 9 + 4 = 4 + 9. You may group any way too.
Grouping means choosing which two numbers to add first. (2 + 3) + 4 = 2 + (3 + 4), and both are 9.
You may multiply in any order: 7 × 3 = 3 × 7.
Turning round means swapping the two numbers. Turning − or ÷ round usually changes the answer. 24 ÷ 8 is 3, but 8 ÷ 24 is 1/3.
For −, the two answers match only when the numbers are equal.
A term keeps its sign when it moves. So 9 − 4 is the same as −4 + 9.
Try the lab to finish this lesson.