y=arctan(x+1)/(x-1） y'=1/[1+(x+1)^2/(x-1)^2]*[(x+1)/(x-1)]' =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)
