# Variable Notation

A variable is a letter or symbol that represents a value that either changes or is unknown. For example, the letter “x” is often used to represent an unknown quantity in an equation or expression. Some variables are commonly used for specific applications. The letters (x, y, z) are often used in the Cartesian Coordinate System to represent space and dimension. The symbol theta (θ) is used in geometry to represent the angle between two lines, and so on.

## Constants

In addition to variables, there are also letters and symbols used to represent constants. Examples of common constants are: the number pi (π), Euler’s number (e), and so on.