f'(x)=[2m(x^2+1)-2x(2mx-m^2+1)]/(x^2+1)^2
=-2[mx^2-(m^2-1)x-m]/(x^2+1)^2
=-2m(x-m)(x+1/m)/(x^2+1)^2,
m>0,
∴-1/m