I also understand

you did know how to use geometry principle to resolve 微积分 question when you were 16 y-old and most chinese kids too. and please remember, my son's school had a few kids their math were much better than my son's in a very normal level public HS. some same year students still learn 5th grade math in this HS. of couse, if you want to compare your kids or best chinese students with this 5th grade level student in HS, whatever you said is right.