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