𝛹 Variables & Expressions

Variables Are Placeholders

A variable (like x or y) is a letter that represents an unknown or changing quantity. An algebraic expression combines variables with numbers and operations: 3x + 2y − 4. There is no equals sign — it's not yet an equation.

Parts of an Expression

• Term: a single piece separated by + or − (e.g., 3x, 2y, −4)
• Coefficient: the number multiplying the variable (3 in 3x)
• Constant: a term with no variable (−4)

To evaluate an expression, substitute numbers for variables and compute: if x = 2, then 3x + 4 = 3(2) + 4 = 10.

Combining Like Terms

Like terms have the same variable part. 3x and 5x are like terms (add coefficients: 8x). 3x and 3y are NOT like terms — different variables. 3x + 5x + 2y − y = 8x + y.