\(\begin{array}{l}
\quad M = A.B\\
\to M = \left(\dfrac{1 - 5\sqrt x}{\sqrt x + 1}\right)\cdot\left(\dfrac{\sqrt x +1}{\sqrt x + 2}\right)\\
\to M = \dfrac{1 - 5\sqrt x}{\sqrt x+2}\\
\text{Xét}\ M - 1\\
= \dfrac{1 - 5\sqrt x}{\sqrt x+2} - 1\\
= -\dfrac{1+6\sqrt x}{\sqrt x + 2} < 0\quad \forall x \geqslant 0;\ x \ne 4\\
\Rightarrow M < 1\\
+)\quad 0 < M < 1\\
\Rightarrow M < \sqrt{M}\\
+)\quad M = 0 \Leftrightarrow x = \dfrac{1}{25}\\
\Rightarrow M = \sqrt{M}\\
+)\quad M < 0\\
\Rightarrow \text{Không tồn tại $\sqrt{M}$}
\end{array}\)
