z=y*cos(x+y)
对x求偏导得y*(-sin(x+y))=-y*sin(x+y)
对y求偏导得cos(x+y)+y*(-sin(x+y))=cos(x+y)-y*sin(x+y)
所以dz=-y*sin(x+y)dx+[cos(x+y)-y*sin(x+y)]dy