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第135页

信息发布者：

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

$- \sqrt {-y}$

$=\frac {\sqrt {20x^2y^2}}{z^5}$

$=\frac {2xy\sqrt {5z}}{z^3}$

$=\frac 1{a}\sqrt {\frac {a+1}{a}}$

$=\frac 1{a^2} \sqrt {a(a+1)}$

$= \sqrt {\frac {48a^2b^2(a+b)}{100}}$

$=\frac 25ab \sqrt {3(a+b)}$

$= \sqrt {27a^2b^3c÷3a^2bc^2×\frac {c^3}{b^2}}$

$= \sqrt {9c^2}$

$=3c$

$=\frac 2{b}×\frac 13×(-\frac 32) \sqrt {ab^5· \frac {a}{b}· a^2b}$

$= -\frac 1{b}\sqrt {a^4b^5}$

$=-a^2b\sqrt b$

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$