View more at http://www.MathTutorDVD.com. In this lesson, you will learn how to add and subtract rational expressions and simply the result. In order to do this, we follow the same rules of fraction arithmetic as we use for regular fractions. Specifically, to add fractions together we must first find a common denominator. When adding rational expressions we must also first find and calculate a common denominator. Once this is done we can add the numerators together as usual and simplify the result.