f(x)=(x+1)/(x-1)=1+2/(x-1)图象是以(1,1)为对称中心，以直线x=1,y=1为渐近线的双曲线。所以，f(x)=(x+1)/(x-1)在(负无穷,0)是减函数uf(u)-f(v)=2/(u-1)-2/(v-1)=2(v-u)/[(u-1)(v-1)]uv-u>0, (u-1)(v-1)>0,f(u)>f(v)
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