#\lim_{x \rightarrow \infty} x^4 + x^5 = \lim_{x \rightarrow \infty} x^4 + \lim_{x \rightarrow \infty} x^5 = \infty + \infty = \infty#.
Or, you could think in this way. #x^5 +x^4# is a polynomial of degree 5. After the last root (the greater one), the function either increase or decrease without a bound. So the limit will always be #\infty# or #-\infty# depending on the sign