In Stewart's Calculus book, the following is said:
A rational function $f(x)/g(x)$ has an oblique asymptote (i.e. $\lim_{x\rightarrow \infty}|f(x)/g(x)-(mx+b)| = 0$) for some line $mx+b, \iff \deg f = \deg g + 1.$
The proof is not provided in the book. Can someone please direct me to a resource that includes this proof?