Expressions using Letter-Numbers - Revisiting Arithmetic Expressions

A variable is a quantity that can take various numerical values; its value is not fixed. We represent variables using letters like $x, y, z, l, m, n$.

Constants are values that remain fixed, such as $5, -10, \frac{1}{2}$.

Algebraic expressions are formed by combining variables and constants using arithmetic operations: addition ($+$), subtraction ($-$), multiplication ($\times$), and division ($\div$).

The value of an expression depends on the value assigned to the variable. For example, if $x = 2$, then $4x = 8$.

We use variables to express general rules in geometry. For instance, if $l$ is the side of a square, its perimeter is $4l$.

Variables are used to express properties of numbers, such as the Commutative Property: $a + b = b + a$ or $a \times b = b \times a$.

The Distributive Property allows us to express $a(b + c)$ as $ab + ac$.