第126页 - 江苏版启东中学作业本八年级数学（上下册）

B

$-\frac{2\sqrt{5}}{5}$

$-\sqrt{-y}$

$ \begin{aligned}解：原式&=\frac{\sqrt{20x^2y^2}}{\sqrt{z^5}} \\ &=\frac{2xy\sqrt{5z}}{z^3} \\ \end{aligned}$

$ \begin{aligned}解：原式&=\frac{1}{a}\sqrt{\frac{a+1}{a}} \\ &=\frac{1}{a^2} \sqrt{a(a+1)} \\ \end{aligned}$

$ \begin{aligned}解：原式&= \sqrt{\frac{48a^2b^2(a+b)}{100}} \\ &=\frac{2}{5}ab \sqrt{3(a+b)} \\ \end{aligned}$

$ \begin{aligned} 解：原式&=a^2\sqrt{b}÷ \sqrt{9a^2b^3} \\ &=a^2\sqrt{b}÷3ab\sqrt b \\ &=\frac{a}{3b} \\ \end{aligned}$

$ \begin{aligned} 解：原式&=\frac{2}{b}×\frac{1}{3}×(-\frac{3}{2})\sqrt{ab^5·\frac{a}{b}·a^2b} \\ &= -\frac{1}{b}\sqrt{a^4b^5} \\ &=-a^2b\sqrt b \\ \end{aligned}$

$解：∵x+y=-6，xy=4$

$∴x＜0，y＜0$

$∴ \sqrt{\frac{y}{x}}+\sqrt{\frac{x}{y}}$

$=-\frac{\sqrt{xy}}{x}-\frac{\sqrt{xy}}{y}$

$=-\frac{\sqrt{xy}(x+y)}{xy} $

$把x+y=-6，xy=4代入$

$得-\frac{\sqrt{xy}(x+y)}{xy}=-\frac{\sqrt{4}×(-6)}{4}=3$