The domain is all x's that can be input into f(x) without f(x) being undefined. Therefore, unless you have an x under a fraction or x^-n, the domain is infinite. If there is a polynomial on the bottom of a fraction, or any expression of x on the bottom, set the bottom equal to zero and that is where the domain is undefined. For example: f(x) = x/(x^2-x-2) f(x) is undefined when (x^2-x-2) = 0. Factor for (x-2)(x+1)=0, then x=2 and x=-1. f(x) is undefined when x=-1 or x=2.