有别于一般文献所使用的构造辅助函数方法,针对在[0,1]上连续,在(0,1)上可导,且满足f(0)=0,f(1)=1的函数f(x),先用反证法可证,存在a,b∈(0,1),使得f′(a)〈1〈f′(b),进而利用导函数的介值性可证,存在ξ,η∈(0,1),使得f′(ξ)f′(η)=1(ξ≠η).
Let function f(x) be continuous on [0,1] with f(0) =0 and f(1) = 1, anddifferentiable on (0,1). It is known that there existξ,η∈(0,1)such that f′(ξ)f′(η)=1(ξ≠η).Different from the use of auxiliary function, we prove the above result by first showing that thereare a,b ∈ (0,1) satisfying f′(a) 〈 1〈 f′(b) ,and then by using the property of the intermediatevalue of the derivative function.