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# Expressions and variables

An algebraic expression is a formula using letters.

$$\Tred{x}+3,\quad 2\times \Tred{n},\quad (\Tred{x}+\Tred{y})^2,\quad 3\Tred{x} - 2,\quad \Tred{t}^2.$$

The letters are called the variables, because they can take several values or inputs. The different values that a letter can take is the range.

The value of the expression is called the output.

To compute the expression, we substitute the letter with its value and we work out the answer.

The value of the expression $$\Tred{x}+3$$ for the input $\Tred{x}=\Tblue{2}$ is the number $\Tblue{2} + 3 = 5$.

The values of an expression can be represented in table.

Table for the expression $\Tred{x}+3$.

 Input Calculation Value $-1$ $0$ $2$ $3$ $5$ $\Tblue{-1}+3$ $\Tblue{0}+3$ $\Tblue{2}+3$ $\Tblue{3}+3$ $\Tblue{5}+3$ $2$ $3$ $5$ $6$ $8$