∫(x,y,)d=im∑R(5,m,5AS) ∑ ∑取上侧,c0sy>0,∴(△S),=(△a)x, 又 (5,7;) i∑R(5,mn,5)(△S)y =im∑R(5,m,(5,m)(△G) 九→ 即「「R(x,y,z)=「Rx,y,x(x,y)d ∑ D = →  =  n i dxdy R i i i Si x y R x y z 1 0 ( , , ) lim ( , , )( )  ( , ) , cos 0, ( ) ( ) , i i i i x y x y z S      =     =    又 取上侧   = → = → =    n i i i i i i x y n i i i i i x y R z R S 1 0 1 0 lim ( , , ( , ))( ) lim ( , , )( )             =  Dxy 即 R(x, y,z)dxdy R[x, y,z(x, y)]dxdy