f(x, y)dy-l(x) 2 因此,VA1,A2>N A A1 f(x, y)dx=l f(x, y)dx-f(x, y)dx <f(x, y)dx-I(x)+f(x, y)dx-I(x <-十 前页)后页)返回前页 后页 返回 ( , )d ( ) , 2 A c f x y y I x  −   因此, 1 2   A A N , , 2 1 2 1 ( , )d ( , )d ( , )d A A A A c c f x y x f x y x f x y x = −    1 1 ( , )d ( ) ( , )d ( ) A A c c  − + − f x y x I x f x y x I x   . 2 2    + = 