(x cos a+ y cosB+z cos yds ∑+∑ =2(x+y+z)dv ∑ J 根据对称性可知「[(x+y)hv=0 Q [J(x2 cosa+y2cosB+z2 cosy)dS=2 fzdv ∑+∑ 2兀 h h =2「a"rdr 0 0 - idz=h 2 反回       1 ( cos cos cos ) 2 2 2 x y z dS   ( x  y)dv  0,      (x cos  y cos  z cos )dS  2 zdv 1 2 2 2 . 2 1 4  h Dxy x y z o 1  h  根据对称性可知        h x y h d rdr zdz 2 2 0 2 0 2    2 (x  y  z)dv