Definition of Variable
The properties of numbers are often written with the help of letters.
For example,we can write the commutative property of addition as .
In this equality we can plug any numbers instead of and : , etc.
Number, that is plugged instead of a letter is called its value.
Letters, that are used in algebra, are called variables; they are called so, because we can change value of a letter.
For example, we can set , or , in equality .
Conclusion. Letters are used to write variables. If it is not a variable (for example, number), then it is called a constant. Value of a constant can't be changed.
Usually, letters and are used for writing variables.
Constants are red, variables are green, operations are blue.