Variable
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).
Variables are very useful in solving many problems algebraicly. For instance, word problems can often be solved using variables to represent desired but unknown expressions.
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.
Problems
Introductory
- What number, when added to
, gives a sum of 
? (Source)
, gives a sum of 
? (
