The theorem is as follow:
Let $a and let $c,d \in [a,b]$ with $c
I am using Shroeder's Mathematical Analysis and I came across this proof, but I think the proof in the book is sort of unclear and therefore I am seeking if there is any other source that has a proof on this theorem. I tried google but they all concern Lebesgue integration of Dirchlet function which is not what I want.