25,
a, A= $\frac{3 }{3x-1}$ $\sqrt{4x^2(9x^2-6x+1)}$
= $\frac{3}{3x-1}$ . 2x$\sqrt{(3x-1)^2}$
= $\frac{3}{3x-1}$.2x(3x-1)
= 6x
B=$\sqrt{\frac{4-√7}{4+√7}}$ + $\sqrt{\frac{4+√7}{4-√7}}$
= $\frac{\sqrt{(4-√7)^2}}{(\sqrt{4+√7+4-√7)} }$ + $\frac{\sqrt{(4+√7)^2}}{(\sqrt{4+√7+4-√7)} }$
= $\frac{4-√7+4+√7}{3}$ =$\frac{8}{3}$
26, ($\frac{1}{x+√x}$ -$\frac{1}{√x+1}$ ):$\frac{√x-1}{x+2√x+1}$
= $\frac{1-√x}{√x(√x+1)}$ .$\frac{(√x+1)^2}{√x-1}$
=$\frac{-(√x-1)}{√x(√x+1)}$. $\frac{(√x+1)^2}{√x-1}$
= $\frac{-√x-1}{√x}$
28,$\frac{a}{\sqrt{ab}+b}$ +$\frac{b}{\sqrt{ab}-b}$-$\frac{a+b}{\sqrt{ab}}$
= $\frac{a(\sqrt{ab}-a)}{(\sqrt{ab}+b)(\sqrt{ab}-a)}$ -$\frac{a+b}{\sqrt{ab}}$
= $\frac{a\sqrt{ab}-a}{\sqrt{ab}(b-a)}$ - $\frac{a+b}{\sqrt{ab}}$
=$\frac{a\sqrt{ab}-a}{\sqrt{ab}(b-a)}$ - $\frac{(a+b)(b-a)}{\sqrt{ab}(b-a)}$
=$\frac{a\sqrt{ab} -a+a^2-b^2}{\sqrt{ab}(b-a)}$
=$\frac{\sqrt{a^3b} -a+a^2-b^2}{\sqrt{ab}(b-a)}$
=$\frac{a-a+a^2-b^2}{(b-1)}$
=$\frac{a^2-b^2}{-(a-b)}$
= $\frac{(a-b)(a+b)}{-(a-b)}$ =-(a+b)=-a-b