Lời giải:
\(\lim\limits_{x\to 4}\frac{\sqrt{x+5}-\sqrt{2x+1}}{x-4}=\lim\limits_{x\to 4}\frac{(x+5)-(2x+1)}{(\sqrt{x+5}+\sqrt{2x+1})(x-4)}=\lim\limits_{x\to 4}\frac{4-x}{(\sqrt{x+5}+\sqrt{2x+1})(x-4)}\)
\(=\lim\limits_{x\to 4}\frac{-1}{\sqrt{x+5}+\sqrt{2x+1}}=\frac{-1}{\sqrt{4+5}+\sqrt{2.4+1}}=\frac{-1}{6}\)