aw a (f+y/ +yf,+yz y2 axa az z OZ M txrf az ou o oy az 125 aa, ou a ov 2+xy2 22 02w 于是 at+x12+y2+yz(1+x") fl+y(x+zf+xyz,+wf, 上一页下一页返回=    x z w 2 ( ) 1 2 f yzf z +    ; 2 2 1 z f yf yz z f    +  +    = =    z f 1 z v v f z u u f       +       1 1 ; 11 12 = f  + xyf  =    z f 2 z v v f z u u f       +       2 2 ; 21 22 = f  + xyf  于是 =    x z w 2 11 12 f  + xyf  2 + yf  ( ) 21 22 + yz f  + xyf  ( ) . 22 2 2 11 12 = f  + y x + z f  + xy zf  + yf 