3.|1f(x,yd f(x,y)do+ f(x, y)d D (D=D1∪D2,D1,D2无公共内点) 如么其分品出为0 工 ①分快 Qa Qh 区分彩   = + 1 2 3. ( , )d ( , )d ( , )d D D D f x y  f x y  f x y 