y=arccos(x+1)/(x-1)
y'=-1/√[1-((x+1)/(x-1))^2] * ((x+1)/(x-1))'
y'=2/√[1-((x+1)/(x-1))^2] * (1/(x-1)^2)
因为：1-((x+1)/(x-1))^2>0
(x-1)^2-(x+1)^2>0,即：-4x>0,x