00- OP 00 OP )dxdy+ )dxdy+ ox d ax ay ax a 5Px+Q+5.Px+Q小+,Px+Q S, Pdx+Ody D2 L2 (L1L2,L3对D来说为正方向)h     −   +   −   +   −   1 2 3 ( ) ( ) ( ) D D D dxdy y P x Q dxdy y P x Q dxdy y P x Q    = + + + + + L1 L2 L3 Pdx Qdy Pdx Qdy Pdx Qdy  = + L Pdx Qdy D1 D2 D3 L L1 L2 L3 ( , ) L1, L2 L3对D来说为正方向