若z = sin ( xy ) 则它的全微分dz =A：xcos (xy) B：(xdx+ydy) cos (xy) C：ycos (xy) D：(ydx+xdy) cos (xy)
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dz=cos(xy)ydx+cos(xy)xdy=(ydx+xdy)cos(xy),选D
======以下答案可供参考======
供参考答案1:
z=sin(xy)
dz = cos(xy) .[ xdy+ydz]
ans:b供参考答案2:
∂z/∂x=cos(xy)*y
∂z/∂y=cos(xy)*x
所以dz=ycos(xy)dx+xcos(xy)dy
供参考答案3:
dz=ycos(xy)dx+xcos(xy)dy