1.az/ax=1/2*1/√ln(xy)*1/(xy)*y=1/(2x√ln(xy))
同理：az/ay=1/(2y√ln(xy))
2.au/am=1/(1+(m^2n)^2)*n*2m=2mn/(1+m^4n^2)
au/an=1/(1+(m^2n)^2)*m^2=m^2/(1+m^4n^2)