In algebra, a variable is usually a letter or other symbol that stands for a number or quantity. Variables are used to represent several mathematical ideas. Some common uses are:

to represent unknowns that do not vary


x + 5 = 8

13 - ? = 4

to represent quantities that vary


The area of any rectangle can be represented by the formula A = l × w.

The area (A) varies as either the length (l) or width (w) of the rectangle varies.

to represent properties


The commutative property of addition can be represented by the equation:

a + b = b + a

For any addends (variables a and b), the order in which they are added does not affect the sum.

3 + 4 = 4 + 3

1 + 7 = 7 + 1

See also algebra.