Rational expressions are fractions with polynomials in the numerator and denominator. The domain of a rational expression excludes values that would make the denominator equal to zero, as division by zero is undefined. To simplify rational expressions, factor the numerator and denominator and reduce where possible. When combining rational expressions through addition, subtraction, multiplication or division, the key steps are to factor denominators, find the common domain, and reduce. Rationalizing techniques include using the conjugate or rationalizing the numerator.