Algebra is known by the use of numbers and letters, an algebraic expression is the combination of both, besides we can use operations like addition, subtraction and so. The following are examples of algebraic expressions:

Letters in an algebraic expression are called variables, when we give specific values to the variables and then look at the result of such expression we say are evaluating the expression. Let us evaluate the first example at and :

We have found the numeric value of that expression. Next example, evaluate at , and