$\lim_{x \to \infty} (4^x+5^x)^\frac{1}{x} = \lim_{x \to \infty} \left(5^x \left((\frac{4}{5})^x +1\right )\right)^\frac{1}{x}\\ =\lim_{x \to \infty} 5\times( )^{\frac{1}{\infty}} \text {(the term inside () ranges between 1 and 2)} \\ =5\times()^0\\ =5\times1\\=5$