几何意义:tana=∫x(x,y),tan月=(x,y) (2)高阶偏导数 az aa ∫x(x,y)=fx(x,y) 2 ax ax ∫(x,y)=∫(x,y) az aaz aray ay、ax fM(x, y)=fyx(, y= a2z a 0z Oyax axon az a a2 ∫m(x,y)=∫m(x,y)=2 avl a K心几何意义: tan f (x, y),tan f (x, y).  = x  = y (2) 高阶偏导数           =    = = x z x x z f x y f x y xx xx 2 2 ( , ) ( , )           =     = = x z x y y z f x y f x y xy xy 2 ( , ) ( , )           =     = = y z y x x z f x y f x y yx yx 2 ( , ) ( , )           =    = = y z y y z f x y f x y yy yy 2 2 ( , ) ( , )