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Properties of addition
Textbook: Part 1, lesson 31, pp. 66–67
GoalLearn the commutative and grouping properties of addition.
New words
commutative property · o‘rin almashtirish xossasigrouping (associative) property · guruhlash xossasiconvenient way · qulay usulflowerpot · gultuvak
Explanation
Commutative property: a + b = b + a. Grouping property: (a + b) + c = a + (b + c). These let us add in a convenient order: first add the numbers that make a whole ten. In a column we add the small number to the big one.
Worked examples
21 + 14 + 39 = (21 + 39) + 14 = 60 + 14 = 74.
36 + 18 + 4 = (36 + 4) + 18 = 40 + 18 = 58.
Class activity
“Handy pairs”: six numbers are written on the board (for example 12, 7, 8, 13, 3, 17). Children find pairs that make a whole ten or twenty and circle them in one colour.
Practice
1
21 + 14 + 39 = ?
74
2
15 + 25 + 24 = ?
64
3
52 + 2 + 8 = ?
62
4
A park has 24 square, 28 oval and 16 round flowerpots. How many in all?
68