I think the series is more difficult-compulsory five (the most difficult problem in general college entrance examination is the series), and then compulsory one function is the most important-(including the trigonometric function of compulsory four)-and it is also difficult to choose the conic curve of 1- 1-For me, compulsory two geometry is the easiest to learn ~ ~ as for compulsory three. In fact, as long as you do more math, nothing will seem too difficult ~ ~ (* _ *) hee hee.