我来试试吧...
z=e^xy*cos(x+y)
Z'x=ye^xycos(x+y)-e^xysin(x+y)
Z'y=xe^xycos(x+y)-e^xysin(x+y)
故dZ=[ye^xycos(x+y)-e^xysin(x+y)]dx+[xe^xycos(x+y)-e^xysin(x+y)]dy