I get that f(x) = P_n(x) + R(x), and calculating P_n(x) is no big deal--it's just a finite Taylor series. But I am struggling with the R(x). Case in point is a function such as f(x) = xe^(x^2). P_n(x) is just x^3 + x. But what the heck do I do with the R(x)? I know it's a big-O of x^4, since that's the next term, but what on earth do I do from there?