# 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.

${\color{red}{{{2}}}}{\color{blue}{{\times}}}{\color{green}{{{x}}}}{\color{blue}{{+}}}{\color{red}{{{3}}}}{\color{blue}{{-}}}{\color{green}{{{x}}}}{\color{blue}{{\times}}}{\color{green}{{{y}}}}{\color{blue}{{+}}}{{\color{green}{{{x}}}}}^{{\color{red}{{{2}}}}}{\color{blue}{{+}}}{{\left({\color{red}{{{3}}}}{\color{blue}{{-}}}{\color{green}{{{x}}}}\right)}}^{{{\color{green}{{{y}}}}}}$

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