记u=x+√v,v=x^2+1
v'=2x
u'=1+v'/(2√v)=1+2x/(2√v)=1+x/√v
则f(x)=lnu
f'(x)=u'/u=(1+x/√v)/u=(x+√v)/(u√v)=1/√v=1/√(x^2+1)