$\begin{array}{l}M = x - 4\sqrt x + 5\\ \to M = x - 4\sqrt x + 4 + 1\\ \to M = (\sqrt x - 2)^2 + 1\\ \to M \geq 0 + 1\\ \to M \geq 1\\ \text{Dấu = xảy ra}\,\,\Leftrightarrow \sqrt x - 2 = 0\Leftrightarrow x = 4\\ Vậy\,\,\min M = 1 \Leftrightarrow x = 4\end{array}$
