Rational expressions are those that have fractional terms. We state restrictions because it may cause the equation to be undefined in some values of x. Undefined questions are imaginary or ideal. There are 7 expressions for undefined terms: these are (∞-∞),∞^∞, N/0, 0⁰, 1^∞, ∞/∞ and 0×∞. The most common restriction for rational expressions is N/0. This means any number divided by zero is undefined. For example, for the function f(x) = 6/x², when you replace x=0, it would result to 6/0 which is undefined. When this function is graphed, you would notice a break at x=0.