y=1/[(x-1)(x+1)]=0.5[1/(x-1)-1/(x+1)]
y'=0.5[-1/(x-1)^2+1/(x+1)^2]
y"=0.5[2/(x-1)^3-2/(x+1)^3]=1/(x-1)^3-1/(x+1)^3