先求偏导数：
zx=ycos(x-y)
zy=sin(x-y)-ycos(x-y)
明显,两偏导数都连续
故全微分存在
dz
=zxdx+zydy
=ycos(x-y)dx+[sin(x-y)-ycos(x-y)]dy
有不懂欢迎追问