A variable is a symbol used to represent either a quantity which is unknown (for example, the solutions of a quadratic equation) or a quantity which may change (that is, which may vary).
Any symbol may be used as a variable, but the most common variables are , and many Greek letters.
By convention, certain variables are often used for certain types of quantities. and are often used as complex variables. and are often used as variables representing integers. and are often used as variables representing prime numbers. And , and are often used as variables representing geometric quantities such as length.
- What number, when added to , gives a sum of ? (Source)