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 `a+b=b+a`.

In this equality we can plug any numbers instead of `a` and `b` : `32+4573=4573+32`, `350+3=3+350` 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 `a=3`, `b=5` or `a=9`, `b=4` in equality `a+b=b+a`.

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 `x` and `y` are used for writing variables.

`huge color(red)(2)color(blue)(times)color(green)(x)color(blue)(+)color(red)(3)color(blue)(-)color(green)(x)color(blue)(times)color(green)(y)color(blue)(+)color(green)(x)^color(red)(2)color(blue)(+)(color(red)(3)color(blue)(-)color(green)(x))^(color(green)(y))`

Constants are red, variables are green, operations are blue.