69
How addition and subtraction are linked
Textbook: Part 2, lesson 66, pp. 54–55
GoalLetter expressions and variables; find the value of an expression by putting a number for the letter.
New words
letter expression · harfli ifodavariable (a letter standing for a number) · o‘zgaruvchivalue of an expression · ifodaning qiymatisum · yig‘indi
Explanation
An expression with a letter is a letter expression, and the letter is a variable – it can stand for different numbers. For example, if x big and t small logs are put on a fire, there are x + t logs. Put a number for the letter and we get the value: in 287 − t + 36 with t = 119 we get 287 − 119 + 36 = 204.
Worked examples
x kg of wood was burned and y kg of ash was left: the ash is x − y kg lighter than the wood.
d + 229 and 229 + d are equal because only the order of the addends changed.
Class activity
“A letter is a hidden number”: write “a + 5” on the board. Children choose a number for a and the class finds the value. Repeat with “a − 3”.
Practice
1
If t = 119, then 287 − t + 36 = ?
204
2
If y = 340, then 635 + y − 220 = ?
755
3
If a = 150, b = 130, c = 180, t = 600, then t − (a + b + c) = ?
140
4
Compare: m + 331 and m + 229.
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