68
How addition and subtraction are linked
Textbook: Part 2, lesson 65, pp. 52–53
GoalNumber expressions with and without brackets, value of an expression, convenient calculation.
New words
expression · ifodavalue of an expression · ifodaning qiymatibracket · qavsscales · tarozi
Explanation
When several additions and subtractions are written with signs, we get an expression. Without brackets: 34 + 25 − 17; with brackets: 78 − (26 − 17). The result of calculating it is the value of the expression. Brackets are done first. Convenient way: (36 + 27) + 24 = 36 + 24 + 27 = 60 + 27 = 87.
Worked examples
(37 + 45) − (38 + 19) = 82 − 57 = 25.
A barrel holds 600 l. On day 1, 235 l were taken and on day 2, 17 l more (252 l): 600 − 235 − 252 = 113 l.
Class activity
“What do brackets change?”: write 50 − (20 + 10) and 50 − 20 + 10 on the board. Children calculate both and explain why the answers differ.
Practice
1
(37 + 45) − (38 + 19) = ?
25
2
(765 − 734) + (420 − 376) = ?
75
3
Calculate in a convenient way: 220 + 34 + 80 + 66.
400
4
A barrel had 600 l of water. 235 l were taken on day 1 and 252 l on day 2. How many litres are left?
113