Letters like x and y stand for unknown numbers.
In x + 3 you add 3 to a number, and the times sign is dropped, so 3 times x becomes 3x.
Just as x candies at 500 won each cost 500x won, one line holds an unset quantity.
Moving the slider to change x shows how x + 3 and 2 times x turn out.
Expressions with Letters
📦 Unknown Numbers as Letters!
①Use letters x, y instead of □ for unknown numbers
②x + 3 means "add 3 to some number"
③Substitute a value to compute the result
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Substituting values for x in expressions
Building Expressions
✏️ Sentences to Expressions
①"5 more than a number" → x + 5
②"3 times a number" → 3 × x = 3x
③"a number minus 2" → x - 2
④Omit ×: 3 × x = 3x
Summary
Variables & Expressions
x + 3, 2x, x - 5
Expressions with unknown numbers as letters
Value of Expression
When x = 4, 2x + 1 = 9
Substitute a number into a letter to evaluate
🎯 Key Points
①Express unknowns with letters like x, y
②Omit ×: 3 × x → 3x
③Substitute to evaluate
④Practice translating sentences to expressions
⑤Foundation for middle-school equations
Examples & Unit Review
Example 1
Write an expression for the price of x candies that cost 500 won each.
1
Multiply the price of one by the number, so write 500 × x.
2
The multiplication sign × can be omitted, so write 500x won.
▸ 500x won
A letter like x stands for a changing number, and the multiplication sign is omitted.
Example 2
Find the value of 2x + 1 when x = 4.
1
Substitute 4 for x: 2 × 4 + 1.
2
8 + 1 = 9.
▸ 9
Replacing a letter with a number to find the value is called substitution.