5）记u=(x+1)/(x-1),则u'=(x-1-x-1)/(x-1)^2=-2/(x-1)^2
y=f'(u)*u'
=-2/(x-1)^2*f'(u)
x=2时,u=3,f'(u)=arctan√3=π/3
故dy/dx=-2π/3