=y+(1+)h(x)+ah +入，Bh2 ow )=x,)+(,)+2'(x,)+0h) =(x,)+h(x,)+ h[过+过 2 ax ay dx」 +Oh) :af+h对+o(h) =y.+y'(x,)+2x+2ay 2+入，=1 02=1/2 B%=1/2 ( ) 2 2 ( ) ( ) 2 ( ) ( ) ( ) ( ) 2! ( ) ( ) ( ) 3 2 2 3 2 3 2 1 O h dx dy y h f x h f y hy x O h dx dy y f x h f y x hy x y x O h h y x y x hy x n n n n n n n n +   +   = +  + +        +   = +  + + = +  +  + ( ) ( ) ( ) ( ) 2 3 2 2 1 1 2 2 y x O h y f h x f y y hy x h n n n n  +   +   + = + +  +            = = + = 1/ 2 1/ 2 1 2 2 1 2     