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设z=ycosu+usiny,其中u=f(x,y)为可导函数,则等于:
A.-ysinu+2usiny
B.-ysinu+cosu+2usiny+ucosy
C.(2usiny-ysinu)f′
D.cosu+ucosy+(2usiny-ysinu)f′
参考答案:D
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