庄由的定义及多元复合函数的求导法则可得 dp(t)=hf(xo+ ht, yo+kt)+kf (o +ht, yo kt ax +ka f(ro+ht, o+kt), ap"(t=hf(xo+ht, yo kt) +2hkf(xo+ ht, yo +kt)+k f(o+ht, y+kt) 上页由 (t) 的定义及多元复合函数的求导法则,可得 ( , ), ( ) ( , ) ( , ) 0 0 0 0 0 0 f x ht y kt y k x h t hf x ht y kt kf x ht y kt x y  + +        +   =  = + + + + + 2 ( , ) ( , ) ( ) ( , ) 0 0 2 0 0 0 0 2 hkf x ht y kt k f x ht y kt t h f x ht y kt xy yy xx + + + + + +  = + +  