71
The associative property of addition
Textbook Part 2, pp. 62–63
GoalKnow that grouping the addends differently does not change the sum, and use it to calculate easily.
New words
associative property · guruhlash xossasitrue / false · chin / yolg‘onconvenient · qulay
Explanation
When adding three numbers we may group them any way: (a + b) + c = a + (b + c). So we first add the pair that makes 10 or a round number: 7 + 8 + 3 → (7 + 3) + 8 = 10 + 8 = 18.
Worked examples
At breakfast the plates had 1 apple + 2 bananas and 3 peaches; today 1 apple + 3 peaches and 2 bananas. The number of fruits did not change: 6.
16 + 9 + 4 → the easy way: (16 + 4) + 9 = 20 + 9 = 29.
Class activity
“True or false” game: the teacher says a statement (“39 has 3 ones and 9 tens”). Children hold up green (true) or red (false).
Practice
1
The easy way: 8 + 7 + 2
17
2
The easy way: 25 + 13 + 5
43
3
True? “The difference of 100 and 50 is 50.”
True
4
Silver is heavier than aluminium but lighter than gold. Which is heaviest?
Gold