I found this as an explanation!
Definite Integral Proof? - Yahoo! Answers
The problem is that I just dont understand it still form the first line:
F(X1) – F(a) = F’(C1) * (X1-a) = F’(C1)*ΔX1 = f(C1)* ΔX1
Please, how do we get this?
p.s. please, don't be angry I ask too much!!!