$\displaystyle \begin{array}{{>{\displaystyle}l}} M=\left(\frac{x+3\sqrt{x}}{x-25} +\frac{1}{\sqrt{x} +5}\right) .\frac{\sqrt{x} -5}{\sqrt{x} +2}\\ M=\frac{x+3\sqrt{x} +\sqrt{x} -5}{\left(\sqrt{x} -5\right)\left(\sqrt{x} +5\right)} .\frac{\sqrt{x} -5}{\sqrt{x} +2}\\ M=\frac{x+4\sqrt{x} -5}{\left(\sqrt{x} +2\right)\left(\sqrt{x} +5\right)} =\frac{x+5\sqrt{x} -\sqrt{x} -5}{\sqrt{x} +2} \ \\ M=\frac{\sqrt{x}\left(\sqrt{x} +5\right) -\left(\sqrt{x} +5\right)}{\left(\sqrt{x} +2\right)\left(\sqrt{x} +5\right)} =\frac{\sqrt{x} -1}{\sqrt{x} +2} \ \\ b) \ Đề\ sai\ nha\ :\ x=\frac{1}{\sqrt{3} -1} -\frac{1}{\sqrt{3} +1} \ mới\ đúng\ \\ x=\frac{\sqrt{3} +1}{3-1} -\frac{\sqrt{3} -1}{3-1} =\frac{2}{2} =1\ \\ Với\ x=1\ thì\ M=\frac{1-1}{1+2} =0\ \end{array}$
