$\begin{array}{l}A = \dfrac{2\sqrt x}{\sqrt x + 3}\\ B = \dfrac{\sqrt x +1}{\sqrt x -3} + \dfrac{11\sqrt x - 3}{x - 9}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(x \geq 0; \, x \ne 9)\\ 1)\,\,x = 25\\ \Rightarrow \sqrt x = 5\\ \text{Thay vào A ta được:}\\ A = \dfrac{2.5}{5 + 3} = \dfrac{10}{8} = \dfrac{5}{4}\\ 2)\,\,M = A + B\\ = \dfrac{2\sqrt x}{\sqrt x + 3} + \dfrac{\sqrt x +1}{\sqrt x -3} + \dfrac{11\sqrt x - 3}{x - 9}\\ =\dfrac{2\sqrt x(\sqrt x - 3)}{(\sqrt x +3)(\sqrt x -3)}+ \dfrac{(\sqrt x +1)(\sqrt x + 3)}{(\sqrt x +3)(\sqrt x -3)}+ \dfrac{11\sqrt x -3}{(\sqrt x +3)(\sqrt x -3)}\\ = \dfrac{2x - 6\sqrt x + x + 3\sqrt x+ \sqrt x + 3 + 11\sqrt x - 3}{(\sqrt x +3)(\sqrt x -3)}\\ = \dfrac{3x + 9\sqrt x}{(\sqrt x +3)(\sqrt x -3)}\\ = \dfrac{3\sqrt x (\sqrt x + 3)}{(\sqrt x +3)(\sqrt x -3)}\\ =\dfrac{3\sqrt x}{\sqrt x - 3}\end{array}$

Giải thích các bước giải:

 a) `x=25`

`=>A=(2sqrt25)/(sqrt25+3)=(2.5)/(5+3)=10/8=5/4`

 b) `M=A+B`

`=(2sqrtx)/(sqrtx+3)+(sqrtx+1)/(sqrtx-3)+(11sqrtx-3)/(x-9)`

`=(2sqrtx(sqrtx-3))/((sqrtx+3)(sqrtx-3))+((sqrtx+3)(sqrtx+1))/((sqrtx+3)(sqrtx-3))+(11sqrtx-3)/(x-9)`

`=(2x-6sqrtx)/(x-9)+(x+4sqrtx+3)/(x-9)+(11sqrtx-3)/(x-9)`

`=(2x-6sqrtx+x+4sqrtx+3+11sqrtx-3)/(x-9)`

`=(3x+9sqrtx)/(x-9)`

`=(3sqrtx(sqrtx+3))/((sqrtx+3)(sqrtx-3))`

`=(3sqrtx)/(sqrtx-3)`