dz=cos(xy)ydx+cos(xy)xdy=(ydx+xdy)cos(xy)，选D
∂z/∂x=cos(xy)*y∂z/∂y=cos(xy)*x所以dz=ycos(xy)dx+xcos(xy)dy
z=sin(xy)dz = cos(xy) .[ xdy+ydz]ans:b
dz=ycos(xy)dx+xcos(xy)dy
