证]y=f(u)可导→im3=f(a) → =f(u+a (im a=0) ∠n→>0 当M≠时,上式化为 4=f(an)·+a.A(1) 当=0时,y=f(u+n)-f(u)=0 (1)式仍然成立 小y u = ac 2021/220 X ∠x ∠x2021/2/20 10 [证] y = f (u)可导     = f (u) + u y (lim 0) 0 = →  u lim ( ) 0 f u u y u =  →    当u  0时,上式化为 y = f (u)u + u (1) 当u = 0时, y = f (u+ u)− f (u) = 0 (1) 式仍然成立！ x u x u f u x y         = ( ) + 