Dear All!

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?

Many thanks!!!

p.s. please, don't be angry I ask too much!!!