y=arctan(x+1)/(x-1）
y'=1/[1+(x+1)^2/(x-1)^2]*[(x+1)/(x-1)]' 复合函数求导法则
=1/[1+(x+1)^2/(x-1)^2]*[(x-1)-(x+1)]/(x-1)^2
=-2/[(x+1)^2+(x-1)^2]
=-1/(x^2+1)