Prove the general idea:
Let f(P) be the sum of any points of partition P. For a partition determined by points, take the maximum and minimum values of functions in each partition respectively, and get M(P) and l (p), which is easy to know, L(P).
l(P ')& lt; = L(P)& lt; = f(P)& lt; = M(P)& lt; =M(P'), integrable.
Swed, you'd better read a book The proof in the book is much more detailed, which is found in general mathematical analysis textbooks.