first i clarify some points that
"most people took 20 mths becoz the 7-mth AP hv limited positions"
altough some very good students of i knew do not take AP exam at all, but in my personal opinion, no one dare to say that if they take AP exam, they will pass.
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based on this point, think you should make good preperation in advance.
what you need to do is:
1> don't forget how to solve maths/physics problems
2> try to solve them in english
"knowing some basic maths/phyics jargons is very useful"
3> Have good command of English
That's all, do not waste time on studying something which you don't know at the moment, but to be very familar with what you know/used to know.
