00 aP 00 aP )dxdy+ 00 aP Oddy+ )dxdy ax a1 D ax a D ax a =,Pdx+Q小+,P+Q小+,P+Q 庄=.P+ah A(L1L2,L2对D来说为正方向)2 上页     −   +   −   +   −   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