##f''(\xi(x))## comes from Taylor's theorem with the Lagrange form of the remainder. This theorem is actually what is used in the original post to prove/derive the error term for the Midpoint Rule (a numerical method of integration):
$$
\frac{h^2(b-a) f''(\xi)}{24}
$$
The value ##\xi## depends...
The screenshot I have attached is actually what is being done (the whole proof, if that's what you meant).
Also, yesterday I have read more and it seems I have understood the actual difference between two versions of MVTI for weaker (integrability only) and stronger case.
When ##f## is not...
I have found the following proof of remainder term for midpoint rule of integration:
and I'm trying to understand the part of it where author is applying MVTI to composition of functions ##f''(\xi_i(x))## and move it out of integral sign as ##f''(\xi_i)##. If we solve Taylor's series for this...