若z = sin ( xy ) 则它的全微分dz =

问题描述:

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)

dz=cos(xy)ydx+cos(xy)xdy=(ydx+xdy)cos(xy),选D