Adding and subtracting a sum
Add a sum to a number, subtract a sum from a number and a number from a sum, writing equalities with brackets as in the Ukrainian Grade 2 book.
- Ukraine: grade 2
- Russia: grade 2
Guide
In Ukrainian Grade 2 children learn expressions with brackets. To add a sum to a number, add one addend and then the other: 7 + (3 + 2) = (7 + 3) + 2 = 10 + 2 = 12. Choose convenient addends so the first number reaches a ten.
To subtract a sum from a number, subtract the addends one after the other: 11 − (1 + 5) = (11 − 1) − 5 = 5. To subtract a number from a sum, subtract it from one addend and add the other: (10 + 6) − 8 = (10 − 8) + 6 = 8.
The book writes the rules with letters: a + (b + c) = (a + b) + c; a − (b + c) = (a − b) − c; (a + b) − c = (a − c) + b. The associative law of addition is (a + b) + c = a + (b + c).