练习+11X二，求f(x)1、已知fxx1+x解：令u.x1则 x :u-l.. f(u)=u? -u+l.. f(x)=x2 -x+1,(x+1)
( ) 1,( 1) 2 f x = x − x + x ( ) 1 2 f u = u − u + 练习 ,( 1) 1 1 , 1 − = = + u u x u x x 解：令 则 1、已知 ，求 x x x x x f 1 1 ) 1 ( 2 2 + + = + f x( )
4x+ 62、已知 f(4x +1)=16x2 +1 ，求f(x)t-1解：设t=4x+1则x4t-1+ 64×t+54即f(t) =(t-1) +1x+5f(x)(x -1)2 +1
2、已知 f ( 4x + 1 ) = ，求 f (x) 16 1 4 6 2 + + x x 解：设 t = 4x + 1 4 − 1 = t 则x ) 1 4 1 16( 6 4 1 4 ( ) 2 + − + − = t t 即f t ( 1) 1 5 2 − + + = t t ( 1) 1 5 ( ) 2 − + + = x x f x