由题意,∑:z=4-(x+y),(x,y)∈D={(x,y)|x2+y2≤1}
∴dS=
1+
z2x(x,y)+
z2y(x,y)dxdy=
3dxdy
∴
∫∫
ydS=
D
3ydxdy=
3
∫2π0dθ
∫10r2sinθdθ=0