?????????????????????????,?????????????????????,????????????????

??????????辭??=5n^2+5n+1990

????????????=5/6*n*(n+1)*(2n+1)+5/2*(n+1)*n+1990n-2000(n>1???????????1???????)

n=20,????53200????????20???????106.4?1????????107?

n=40,????192400,??????40???????384.8?1????????385?

????????????????B??????????6???????????2?20?????飬??????????????220??????????????A????????ü?????????ù???????? ??????C????????? ????????????????????????……????????????????? ???ж??????????

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