00 dxdy=h, 2(x, y)dy ax 同理可证-,小=P(xy D 两式相加得 oo aP )dxdy=t Pdx+ody (2)D由一条分段光滑的闭曲线围成, D 将D分成三个既是X型又是 Y一型的区域D1,D2,D3 a0 aP )x= a0 aP )dxdy D1+D,+D. ax ay =    L D dxdy Q x y dy xQ ( , ) 同理可证 ( , ) .  =   − L D dxdy P x y dx yP 两式相加得   = +  −  L D dxdy Pdx Qdy yP xQ ( ) (2) D由一条分段光滑的闭曲线围成, 将D分成三个既是X −型又是 Y −型的区域D1 ,D2 ,D3 . L L1 L2 L3 D D 1 D 2 D 3   + +  −  =  −  1 2 3 ( ) ( ) D D D D dxdy yP xQ dxdy yP xQ