It is so confusing for this chapter as it seems that the step taught here is not applicable in some other question. May I know what is the correct way to take note when tackling similar questions?
I really don’t know what you mean – have you watched the lecture that goes with the notes (because it is in the lectures that we expand and explain – the notes should not be used on their own)?
What is covered in the lecture are the only techniques that have been needed in every F9 question on this topic that has ever been asked (most usually – as the lecture makes clear – the techniques covered in examples 2 and 3 of the chapter).