求导法则
?2 求导法则
P.103习
,,,4(对下列各函数计算 f(x),f(x,1),f(x,1)
333f(x),xf(x,1),xf(x,1),x(1) (2) (3)
222,,,f(x),3xf(x,1),3(x,1)f(x,1),3(x,1)解 (1),,
322,,f(t),(t,1)f(t),3(t,1)f(x),3(x,1)(2)令,则,于是。从而,x,1,t
2222,,f(x,1),3(x,1,1),3xf(x,1),3(x,1,1),3(x,2),
32,f(x,1),xxf(x,1),3x也可以如下进行:在两端分别对求导数,得,再令
,……. x,1,t
322,,f(t),(t,1)f(t),3(t,1)f(x),3(x,1)(3)令,则,于是。从而,x,1,t
2222,,f(x,1),3(x,1,1),3(x,2)f(x,1),3(x,1,1),3x,
dd226(设f为可导函数,证明:若时有, f(x),f(x)x,1dxdx
,f(1),0f(1),1则必有或
dd222,,证明 因为,,所以当时有f(x),2xf(x)f(x),2f(x)f(x)x,1dxdx
dd22,,,,f(x),2f(1)f(x),2f(1)f(1)2f(1),2f(1)f(1),,由题设,有,dxdxx,1x,1
,,f(1)(1,f(1)),0f(1),0f(1),1于是,从而或
另证:当时 x,1
2222df(x)f(1)f(x)f(1),,2f(x),lim,lim(x,1)2x,1x,1dxx,1x,1x,1 f(t)f(1),,,lim,lim(x,1),2f(1)t,1x,1t,1
22df(x),f(1)f(x),f(1)2f(x),lim,lim(f(x),f(1))x,1x,1dxx,1x,1x,1 f(x),f(1),,lim,lim(f(x),f(1)),2f(1)f(1)x,1x,1x1,
,,,,2f(1),2f(1)f(1)f(1)(1,f(1)),0f(1),0f(1),1由题设,有,于是,从而或