In summary, a rational expression is a mathematical expression that contains a fraction with polynomials in the numerator and denominator. To subtract them, a common denominator must be found. The expression 1/2x-7/x-3 can be simplified, but is undefined if x=0. Rational expressions with different denominators cannot be added or subtracted without first finding a common denominator. A rational expression differs from a rational number in that it is an algebraic expression, while a rational number can be expressed as a fraction of two integers.