giúp mk với

Bài 1: a) Ta có: $A=\sqrt{9+4\sqrt5}-\sqrt{9-4\sqrt5}$ Đặt $\sqrt{9+4\sqrt5}=a$ và $\sqrt{9-4\sqrt5}=b$ Ta có $a^2=9+4\sqrt5$ và $b^2=9-4\sqrt5$ Từ đó suy ra $a^2+b^2=18$ và $ab=\sqrt{(9+4\sqrt5)(9-4\sqrt5)}=\sqrt{81-80}=1$ Vậy $A=a-b=\sqrt{a^2+b^2-2ab}=\sqrt{18-2}=4$ b) Ta có: $B=\sqrt{x^2-10x+25}$ Với $x< 0$, ta có $x^2-10x+25=(x-5)^2$ Vậy $B=\sqrt{(x-5)^2}=|x-5|=-(x-5)=5-x$ Bài 2: a) Ta có: $(\sqrt8+\sqrt{18}+\sqrt5)(\sqrt{50}-\sqrt5)=\sqrt{8\times50}+\sqrt{8\times(-5)}+\sqrt{18\times50}+\sqrt{18\times(-5)}+\sqrt{5\times50}-\sqrt{5\times(-5)}$ $=20+2\sqrt{90}+30-3\sqrt{90}+5-5=55$ b) Ta có: $\frac{\sqrt3-1}2+\frac{\sqrt3+2}3-\frac{2\sqrt3-1}4=\frac{2(\sqrt3-1)}4+\frac{4(\sqrt3+2)}{12}-\frac{3(2\sqrt3-1)}{12}$ $=\frac{\sqrt3-1}{2}+\frac{\sqrt3+2}{3}-\frac{2\sqrt3-1}{4}=\frac{4\sqrt3+4}{12}+\frac{4\sqrt3+8}{12}-\frac{6\sqrt3-3}{12}$ $=\frac{8\sqrt3+12}{12}=\frac{2\sqrt3+3}{3}$ Bài 3: Ta có: $\frac{4\sqrt x+2}{7\sqrt x-2}=\frac23$ $\Leftrightarrow 3(4\sqrt x+2)=2(7\sqrt x-2)$ $\Leftrightarrow 12\sqrt x+6=14\sqrt x-4$ $\Leftrightarrow 2\sqrt x=10$ $\Leftrightarrow \sqrt x=5$ $\Leftrightarrow x=25$ Bài 4: a) Ta có: $P=(\frac{\sqrt x+1}{\sqrt x}-\frac2{\sqrt x+1}):(\frac{\sqrt x}2+\frac1{2\sqrt x})$ $=\frac{(\sqrt x+1)(\sqrt x+1)-2(\sqrt x)}{\sqrt x(\sqrt x+1)+\sqrt x}$ $=\frac{x+2\sqrt x+1-2\sqrt x}{x+2\sqrt x+\sqrt x}$ $=\frac{x+1}{x+3\sqrt x}$ b) Đặt $x=3-2\sqrt2$, ta có: $P=\frac{(3-2\sqrt2)+1}{(3-2\sqrt2)+3\sqrt{3-2\sqrt2}}$ $=\frac{4-2\sqrt2}{3-2\sqrt2+3\sqrt{3-2\sqrt2}}$ $=\frac{4-2\sqrt2}{3-2\sqrt2+\sqrt{12-8\sqrt2}}$ $=\frac{4-2\sqrt2}{3-2\sqrt2+\sqrt{(2-\sqrt2)^2}}$ $=\frac{4-2\sqrt2}{3-2\sqrt2+2-\sqrt2}$ $=\frac{4-2\sqrt2}{5-3\sqrt2}$ $=\frac{(4-2\sqrt2)(5+3\sqrt2)}{(5-3\sqrt2)(5+3\sqrt2)}$ $=\frac{20-6\sqrt2+12\sqrt2-6}{25-18}$ $=\frac{14\sqrt2+14}{7}$ $=2\sqrt2+2$ c) Ta có: $\frac{x+1}{x+3\sqrt x}=1$ $\Leftrightarrow x+1=x+3\sqrt x$ $\Leftrightarrow 1=3\sqrt x$ $\Leftrightarrow \sqrt x=\frac13$ $\Leftrightarrow x=\frac19$ Bài 5: a) Ta có: $P=\frac3{\sqrt x}+(\frac x{x-\sqrt x}+\frac{x+1}{\sqrt x}-\frac1{\sqrt x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{x(x+\sqrt x+1)+(x+1)(x-\sqrt x)}{(x-\sqrt x)(x+\sqrt x+1)}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{x^2+x\sqrt x+x+x^2-x\sqrt x-\sqrt x}{x^2-x}.\frac{\sqrt x}{x+\sqrt x+1})$ $=\frac3{\sqrt x}+(\frac{2x^2+x}{x^2-x}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x^2+x}{x(x-1)}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).\frac{\sqrt x}{x+\sqrt x+1}$ $=\frac3{\sqrt x}+(\frac{2x+1}{x-1}).