4.设f(x)在[a,b]上连续,(a,b )内可导, f(a)=a , (overrightarrow (b))=b, 且 in [ a,b] 时 (x)neq 0. 证明:至-|||-少存在一点 xi in (a,b), 使得 (xi )=xi cdot f'(xi )