au au au 例21设u=f(x+y+x,x2+y2+z2),求 OX OV 02 解令s=x+y+z,t=x2+y2+z2 则=f(s,1),从而是x,y,的复合函数 Ou of as of at of of ax as ax at ax as at dS OS az as az at az as at6 2 2 2 21 ( , ), , , . uuu u f x y z x y z x y z  = + + + +    例 设 求 则 从而 是 的复合函数 u f s t u x y z = ( , ), , , . y u s t x z u f s f t x s x t x      =  +       2 f f x s t   = +   u f s f t y s y t y      =  +       2 f f y s t   = +   u f s f t z s z t z      =  +       2 f f z s t   = +   2 2 2 解 令 , , s x y z t x y z = + + = + +