商的微分法则，分子应该是减号不是加号
y= x/√(x^2+1)dy =dx/√(x^2+1) + x. d [1/√(x^2+1)]=dx/√(x^2+1) + x. [-x/(x^2+1)^(3/2)] dx= [(x^2 +1 -x^2)/(x^2+1)^(3/2) ] dx= [1/(x^2+1)^(3/2) ] dx
