71
Inverse operations
Textbook: Part 2, lesson 68, pp. 58–59
GoalInverse operations: subtraction undoes addition and addition undoes subtraction.
New words
inverse operation · teskari operatsiyastarting state · dastlabki holatthe number thought of · o‘ylangan sonreturning, undoing · qaytarish
Explanation
An operation that returns an object to its starting state is called an inverse operation. The inverse of adding is subtracting, and the inverse of subtracting is adding. That is why 789 − 300 + 300 = 789 and a + 11 − 11 = a. Jasur thought of a number, added 57 and got 78. To find his number use the inverse: 78 − 57 = 21.
Worked examples
563 + 220 − 220 = 563: we added and then took away the same amount.
(y − 211) + 117 = 658: y − 211 = 658 − 117 = 541, so y = 541 + 211 = 752.
Class activity
“Guess my number”: a child thinks of a number, adds (or subtracts) a number and tells the result. A friend finds the original number with the inverse operation.
Practice
1
457 − 223 + 223 = ?
457
2
168 + 348 − 348 = ?
168
3
Jasur thought of a number, added 57 and got 78. What number did he think of?
21
4
700 kg of coal came for heating. 26 kg were used on day 1 and 7 kg more (33 kg) on day 2. How many kg are left?
641