38
The commutative property of addition
Textbook Part 1, pp. 78–79
GoalKnow that swapping the addends does not change the sum, and use it to calculate.
New words
swapping places (commutative) · o‘rin almashtirishproperty · xossadoes not change · o‘zgarmaydi
Explanation
Swapping the addends does not change the sum: 3 + 7 = 7 + 3. This is handy: instead of 2 + 8 we work out 8 + 2 — counting on from the bigger number is easier.
Worked examples
Salim borrowed 4 textbooks and 2 storybooks: 4 + 2 = 6 and 2 + 4 = 6.
1 + 9: start at 9 and count on 1 → 10.
Class activity
Two paths on a ruler: 3 + 7 (7 steps from 3) and 7 + 3 (3 steps from 7). Both land on 10. Children invent their own example and get a partner to check it.
Practice
1
2 + 7 = 7 + _
2
2
Work out the easy way: 1 + 8
9
3
Is it true? 5 + 4 = 4 + 5
Yes, both are 9
4
Write 5 natural numbers in a row starting from 3.
3, 4, 5, 6, 7