if u ask ur teacher " eh. why sin 2A = 2sinAcosA lidat har? ", i
think he/she wouldnt reply you. haha. for A maths trigo, all you
need is mere memorise + practice. try to attempt as many questions
as possible. it does help, seriously.
then again, if you have doubts, ASK whoever knows the
chapter well. For A maths, practice is just the only solution to
it.