In addition I would like to know that why marginal revenue = differentiation of total revenue with respect to differentiation of total quantity? What does differentiation mean?
Have you watched my lectures on this (on this website), because the lectures explain it.
Also, the notes (and my lectures on here) make it clear that you cannot be expected to differentiate (differentiation is not in the syllabus and so you do not need to know about it). Differentiation is not in the syllabus for any of the ACCA exams.