解：因?yàn)?$\frac {xy}{x+y}=-2$?
所以?$\frac {x+y}{xy}=\frac {1}{x}+\frac {1}{y}=-\frac {1}{2}$?
同理可得：?$\frac {1}{y}+\frac {1}{z}=\frac {3}{4}，$??$\frac {1}{x}+\frac {1}{z}=-\frac {3}{4}$?
所以?$\frac {1}{x}+\frac {1}{y}+\frac {1}{z}=(-\frac {1}{2}+\frac {3}{4}-\frac {3}{4})÷2=-\frac {1}{4}$?
所以原式?$=\frac {1}{\frac {1}{x}+\frac {1}{y}+\frac {1}{z}}=-4$?
