令u=x^y v=y^x
dz/dx=dz/du*du/dx+dz/dv*dv/dx
=df/du*y*x^(y-1)+df/dv*lny*y^x
dz/dy=dz/du*du/dy+dz/dv*dv/dy
=df/du*lnx*x^y+df/dv*x*y^(x-1)